I just finished reading this and I'm not sure what to put down. It would be most polite to say "What a delightful story!" and "Very clever plot" but the question that keeps barging to the forefront of my brain is "Did you just not want to warn for ungulate sex, or did you actually not want to write it? Because I feel pretty sure you could." ;-)
Seriously though, that was a charming story. When I saw the title, I figured they were going to be transfiguring themselves into different animals (like the song goes) but I never expected what you did, which was in fact very clever, and a nice twist. I'm awfully picky about the Snarry I read, but Obsessed!Harry pursuing a reluctant, vile-tempered, cranky, get-offa-my-lawn Snape is probably my favorite type, and this is a wonderful example. It really is a delightful story and I enjoyed it immensely.