One horse has died in a fire at allotments in Sunderland, while another was so badly slashed that it had to be put down, animal rescue workers said.
The fire, which is being treated as arson, happened at the city's Downhill Road Allotments at about 01:00 BST.
When RSPCA staff arrived they found a Shetland pony called Murphy in a "very distressed" state after being stabbed and slashed at least seven times.
The wounds were so bad that the animal had to be put down.
Police are investigating the incident.
RSPCA inspector Terri Ann Fannon said: "It was an absolutely terrible thing to have to deal with. We were told that one horse died in the fire.
"Murphy was in an awful state. He was very distressed, as you'd expect.
"He had numerous slash and stab injuries all over his body. One stab wound on his back left knee was so deep the damage was irreparable.
"Very sadly the vet decided that there was no other option but to put him to sleep."
No-one has so far claimed ownership of the animals.
RSPCA inspector Trevor Walker, added: "Clearly this was a deliberate attack. It's absolutely shocking that someone would do something like this."
The charity and Northumbria Police have appealed for witnesses.