lameness all my horses are going stiff on there legs

[manch12]

Hi can anyone please help all of my horses are coming down with very stiff legs joints its not laminitus as they have all come down with it one day they are great trotting around and then bang one by one they are so stiff they just stand there and they are in so much pain they can't move its been over the last 6 months I've had to have 3 horses put Down it got so bad for them now my too Shetlands have just come down with it took me an hour to move them 10 feet today too days ago they where fine can anyone please help thank you.I've not changed any of there food nothing has changed in there lifestyles at all just Dont no what the problem is.

[Caramel custard]

What did the vet diagnose when you had your horses pts?