[kanawa]

I don't think it is found among animals...

I only know what I've seen.
It is strange to see a female dog mount another female and go through the same motions as a male dog.
But I don't see any indication that it is done for pleasure but rather as means to establish dominance and submission.
Likewise human homosexual behavior probably has it's roots in similiar motivations with the added human twist of lust for pleasure.

[Rudder]

Actually, I studied dog sexual behavior as part of my duties in grad school. We had two large packs of over 100 dogs in each; also wolves and assorted other animals. The mounting and thrusting behavior by females is typical for many species (dogs, rats, mice, cows) but what is mounted doesn't matter to the mounter. In the case of males, males will mount anything, but usually the mountee (sorry RCMP), if a male, will just walk away, sometimes in a huff and a growl. I never saw anything like true homosexual behavior (wherein the preferred partner was of the same sex) in any animal, except, of course homo.