Oh! I must be out of tune with puzzling like this. Could not fathom it, even when presented with the solution.

3 of the groups work. However, the Billy Goat group is just wrong... a Billy Goat is ... erm... a GOAT! As is a Kid. Goats and sheep are different species. Merino is a breed of sheep, so the only one that actually fits your solution is Ram.

FirstGofer: you're right, I screwed up here. In the way that I compile and put together potential responses, I made an error and rushed this one through. Och! Sorry. Wish I could take it back, but what can you do?

relieved about the goat issue, i cdnt get it. also clever pronoun one- i missed it but was spending too long on the "sheep" maybe. best puzzle creator so a mistake on his/her side instead of mine is nice to see!

edc: Thanks for your kind words, thanks for playing, and most importantly thanks for your patience!