Giant armadillo (Priodontes maximus)

You think you'd notice something the size of a large pig, covered in scales with 20cm (8") front claws like scimitar blades. But the giant armadillo does such a good job of hiding, scientists have had to install secret cameras to find out more about it.

"Very few people have seen a giant armadillo in the wild," says Arnaud Desbiez, who runs the Giant Armadillo Project in Brazil. "In our field site the owner of the ranch, who was born and raised here, had never seen a giant armadillo before we started the project."

Weighing up to 50kg (110 lb) and reaching 1.5m (5 ft) in length, it is the largest species of armadillo.

At nearly twice the size of other species, its girth makes it unable to roll into the family's distinctive ball-shape for defence. Instead, it digs underground burrows with its impressive front claws. It only ventures outside under cover of darkness.

The giant armadillo is considered a vulnerable species due to habitat loss and hunting, but local people are said to regard sightings of them as bad omens. The rare camera trap photographs may help to highlight its situation.

Giant squid (Architeuthis)

Possibly the most infamous of giant animals lurks beneath the waves. The giant squid earns its name from a body size of up to 5m (16.5 ft) and a pair of elongated tentacles that can bring it to an overall length of 13m (42.5 ft).

It is a predator, known for its huge flesh-ripping beak and eyes the size of footballs. But as a denizen of the deep ocean, living as far down as 1000m (3,280 ft), it is a species that very few people have seen alive.

Salty sea tales claim that monstrously large animals have destroyed ships, but documented encounters are rare. Most take place on the water's surface, when the squid is injured or dying.

The first footage of a giant squid in its natural deep-water environment was filmed in 2012.

The project was organised by a team of international scientists who launched a submersible vehicle off the coast of Japan. With a bit of bait and a lot of luck, an animal appeared in front of their cameras.

Giant otter (Pteronura brasiliensis)

While it lacks elephants, tropical South America is still a land of giant animals. The continent is home to the biggest members of the armadillo and anteater families, as well as the capybara, the world's largest rodent.

In the rivers east of the Andes lives the giant otter. Twice the size of the next biggest member of its family, the giant otter can reach 2m (6.5 ft) in length.

It lives in open habitats in large family groups, and is consequently quite easy to spot.

While giant otters can deal with natural predators such as caiman and jaguars, they have fallen victim to man. They are described as sociable and curious, and this gregarious character has made them a target for hunters.

The thick pelt was once highly sought after, with devastating consequences. The trade was banned in 1975 but now the remaining animals are threatened by increasing human settlement in their Amazonian habitat.

Conflict arises with fishermen, and mismanaged tourism is also having an impact, according to the International Union for the Conservation of Nature. Populations in the protected Pantanal wetland are thought to have recovered, but it remains to be seen how the otters will cope with the rising human population.

Giant huntsman spider (Heteropoda maxima)

If you measure spider size by leg span, the largest reaches 30cm (1 ft) across and goes by the worrying name "giant huntsman spider". Fortunately, it confines its predatory activities to insects.

You're unlikely to see one scurrying across your carpet, unless you've set up home in a cave in Laos. Even there it would now be a rare sight.

Heteropoda maxima made headlines among arachnophobes and -philes alike when it was discovered in 2001 by Dr Peter Jaegar of Johannes Gutenberg-University in Mainz, Germany.

Jaegar says the resulting attention has been bad news for the spider, due to unregulated demand by the pet trade. He suggests that for every 100 spiders imported as pets, 1000 more may have perished when removed from their homes.

"In 2009, my PhD student and I could observe the impact in easily-accessible caves, where no adults could be found," says Jaegar. He says the spiders survive for little more than a year outside of the controlled climate of their caves.

Jaegar says their short-lived nature may ultimately reduce demand.

Giant oarfish (Regalecus glesne)

Looking very much like an enormous sea serpent, the giant oarfish is extremely flat and undulates through the water like a wide silver ribbon.

Its long pelvic fins look like oars and it has a distinctive red crest. But the most remarkable thing about the oarfish is its extreme length. Measured at up to 17m (56 ft), it is the world's longest bony fish.

Despite its massive size, the giant oarfish remains quite mysterious. It lives in the deep ocean alongside other giant species. It's not clear why deep-dwelling species are often so big: low temperatures, high pressure, a lack of currents or a scarcity of food have all been put forward as explanations.

Because of its deep-dwelling habits, the giant oarfish is a rare sight. In recent years, unmanned submersibles have managed to film it in its natural environment, but very few people have seen a healthy one in the flesh.

Giant oarfish only appear at the surface when dead or injured. Where one washes in to shore, it is usually photographed being held aloft by large groups of people to show its relative size.

In June 2015, Santa Catalina Island off the coast of California hosted its third giant oarfish in as many years when a 5.2m (17 ft) specimen washed ashore.

Goliath frog (Conraua goliath)

The world's largest frog can weigh as much as a newborn baby, tipping the scales at 3.2kg (7lbs).

The goliath frog might be huge, but like many of its amphibian cousins it does its best to conceal itself. Mottled green camouflage helps it to hide among the moss-covered rocks.

It lives in or near fast-flowing rivers in the coastal rainforests of west Africa.

You might think it would have an impressive croak to match its bulk, but you'd be wrong. Unlike most other frogs, the goliath frog does not possess a vocal sac, so it whistles to attract mates instead.

Despite its stealthy adaptations, the goliath frog is an endangered species.

Its population has reportedly declined by 50% in the last three generations. The frogs are widely hunted as a delicacy and for the international pet trade, notably for frog-jumping competitions in the US.

While some frogs have been exported for captive breeding programmes, these have so far proven unsuccessful. Instead conservationists are concentrating on local communities, in a bid to curb unsustainable hunting.

Chan's megastick (Phobaeticus chani)

While most insects can fit in the palm of your hand, there are a few giants out there. The longest in the world is a stick insect that lives in Borneo. It was named after its discoverer as Chan's megastick in 2008.

The largest known example is over 0.5m (22") long with legs outstretched and is kept at the Natural History Museum in London, UK.

Very little is known about the insect because in the wild it is exceptionally difficult to see. Males are brown and females mottled green, and both are long and spindly, so they are perfectly camouflaged in their rainforest home.

To further mimic the plants around them, the eggs of the insect resemble seeds and have wing-like extensions, which are thought to help them to disperse on the wind.

Experts believe Chan's megastick lives in the forest canopy, making it even more challenging to find. Only a handful of specimens are currently known to science.

Queen Alexandra's birdwing (Ornithoptera alexandrae)

In Papua New Guinea, there is a butterfly so large it earns comparison with a bird.

The Queen Alexandra's birdwing lives in a small area of rainforest near the north coast.

Males are spectacular, with blue-green iridescent streaks on velvet black wings and a bright yellow abdomen, while the females are slightly more reserved with cream accents. However females are a third larger than males, with a record-breaking wingspan of up to 30cm (1 ft).

Following its initial discovery in 1906, the insect was highly prized by collectors and as a result it was severely over-harvested. It flies fast through the rainforest canopy and proved difficult to capture, so hunters used shotguns loaded with salt to bring it down.

A law was introduced in 1966 to protect the butterfly, but continued illegal collection and habitat destruction for logging and palm oil has dramatically decreased its population.

Few outside of Oro province have seen the endangered Queen Alexandra's birdwing in flight and even there sightings are dwindling. Local tribes and conservationists are now fighting for its future.

Giant isopod (Bathynomus giganteus)

Imagine a woodlouse that can grow longer than a cat, 76cm (2.5 ft) long – and weigh 1.7kg (3.75 lb). Well, it exists and it's called the giant isopod.

It is a crustacean, distantly related to shrimp and crabs. It lives far beneath the waves where growing to relatively giant proportions is not uncommon.

This oversized roly-poly shares its terrestrial cousins' rigid exoskeleton and ability to roll into a ball for defence. It has seven pairs of legs, two sets of sensitive antennae and large compound eyes. It is a pale lilac colour.

In cold waters off the US coast it inhabits the sea bed, feasting on the corpses of whales, fish and squid. Food is a rare resource when you live as far down as 2000m (6560 ft) below sea level so when this scavenger finds a meal it attacks.

Its targets can include fishermen's trawl nets, and most encounters with the creature occur when it is dragged to the surface as bycatch. In 2010, a huge specimen was found latched onto a remotely-operated vehicle being used in sea surveys.

Such wild encounters with the animal are rare, but several are held in aquarium collections around the world, and they are particular favourites in Japan.

Blakiston's fish owl (Bubo blakistoni)

There is some debate over which owl gets to lift the title of world's largest, but the Blakiston's fish owl is a heavyweight contender. It reaches up to 4.6kg (10 lb) in weight with a wingspan nearing 2m (6.5 ft).

It was discovered by naturalist Thomas Blakiston in 1883. As its name suggests, the massive owl feeds primarily on fish.

Living in riverside forest in Siberia, northeast China, North Korea and northern Japan, it cuts a bulky figure among the tree tops.

But this is now a rare sight. Under increasing pressure from logging, overfishing and persecution by hunters, Blakiston's fish owl is now officially considered an endangered species.

In Hokkaido, Japan, the owl was traditionally considered a spirit that protected the villages of the indigenous Ainu people. Now, roles are reversed and conservationists watch over the owls with the help of nest box cameras.

Thanks to these huge artificial homes, the decline of owls has been halted. But without the mature forests they depend on, their future is unclear.