My Pony has a bony lump on his back pastern, what is it?

[Shrek-Eventing-SW]

My pony has recently got a bony lump on his back pastern. It came up suddenly and he has not been lame on it. He has splints on his front legs, and it is just like a splint. Any ideas to what it could be? Thank you

 

[Booga22]

Is he atall lame or any heat associated with it?

 

[Shrek-Eventing-SW]

Not lame and no heat. Thats why I can't figure out what it is.

 

[Louby]

Maybe its a knock, just like a splint?? Just with it coming suddenly
My old mare had these but on all 4 legs, they grew slowly at first and looked like golf balls on the outsides of each pastern. Hers were ringbone but they didnt appear overnight.