Reptiles don't do it for me, although I'm intrigued by the masturbating turtle videos and some male 'gator pics and vids I've seen. Birds: Ostriches and Emus as others have mentioned are mildly interesting in images; the few that I've seen up close and personal were outright scary.
Horses: When I was much younger, I reached into a gelding's sheath and felt up his penis a bit. He didn't seem to mind, but my hand came out caked with nasty, sticky, smelly stuff. I don't know much about horses other than they can hurt you very badly if you don't know exactly what you're doing. I believe you pretty much have to grow up around them. Other large animals (cows, etc.), the same applies. However, I went with a friend to look at a car where the people had a really sweet jennet (she-donkey) who I made friends with. I considered visiting her some night, but (no doubt wisely) chickened out.
Monkeys and apes really creep me out. Some people have fantasies about them, not me.
I'm essentially a dog guy, but in the past I've played with some house cats and small mammals. I once had a male Guinea pig (a.k.a. cavy) who would face-fuck me and unload in my mouth pretty regularly. That was nice, but beyond that small-and-furries are only good for petting and looking cute.
Pigs? I've made friends with one or two but never saw them as sexy. I have to admit, though, their junk is fascinating.
I'm old and set in my ways, so I suppose I'll just stick with what I know and any other critter that I find sexy will just have to stay on the computer screen.