Penguins. There's a pair of male penguins at a zoo (I THINK it's the national zoo, but don't quote me on that) that are obviously homosexual. In fact, they do everything, including raising an orphaned chick, together.
Most of the higher primates have been observed engaging in homosexual activity.
It's a lot more common than most people think, and it makes the "they chose to be homosexual" argument that much more stupid - after all if we're going to say that homosexuality is a choice then we have to admit that animals have much greater intelligence than we have previously given them credit for
