Donalewis, domestic cattle have been in the UK since 4000 BC, chances are the farmer can't remember that far back unless he's an Irishman. Realistically the tooth you found could be several hundred years old.
WHAT ...Your horses BABY TOOTH ...I didn't even know there were such things !
Good job Red gave you a solid answer to the found tooth, I brought an older posting up to this date, look for "LARGE TOOTH FOUND > > > > " Lots of input but a real fast look ID is tricky.
There's a lot of neat antiquity on your little isle.." Keep looking down "