Evolutionary biologist here. I’d argue that, in the same sense as we see homosexuality in animals, we see trans animals.

Some animals physically transition - there are fish that will change their physical gender based on the current gender mixture in their local environment. Some behaviorally transition, with males taking on female roles. Sometimes a whole species is trans - like the female hyena developing male appearing genitalia.

Sexual orientation in the animal kingdom is not strictly analogous to that among humans (which has a much stronger social construct), and the same is true of gender (that is, human gender is a social construct). Because the range of adaptations are so diverse and so widespread, I’m very sure of the fact that they have different causes from each other as well as from humans, but the same is true of animal sexuality.

birds sometimes act as if they were opposite sex