Yes! There are instances of exclusively homosexual animals in almost every species! There is a book I have called "Biological Exuberance: Animal Homosexuality and Natural Diversity", and I recommend that EVERYONE flip through it at some point. It was a HUGE eye-opener for me!!

I'll pull out info about dogs that I find in there....*looking*

"Same-sex pair bonding as well as homosexual mounting (including ejaculation between males) had been documented in Dogs, including breeds such as Beagles, Basenjis, Cocker Spaniels, and Weimeraners. Examples of both female and male homosexual behavior in Cats have also been confirmed, including mutual genital stimulation and mounting among females, and mounting leading to orgasm among males..."

There's a little more on domestic animals in there, but the vast majority of the book is about wild animals. I'm BLOWN AWAY by how many species form life-long homosexual pair bonds! One that sticks out in my mind is flamingos:

Two male flamingos will mate for life. They build a nest together with the rest of the flock, and a female of their flock will come lay eggs in their nest, then move back to her own! The males take care of the chicks, and the female gets to spread more of her genetic materials! (Most of the chicks turn out to be heterosexual)

You can find homosexual behavior in fish, birds(LOTS of birds actually!), reptiles, rodents, seals, pretty much EVERY animal on earth! It's not uncommon at all, it's just not documented much.

I could go forever, but I'll stop for now. If you're interested, PM me.