That doesn't really make sense to me. That's sexual variance, not gender variance.
And it would be kinda silly to call plants "transsexual" when their normal sexual behavior is not a male-female binary. I just don't see how it relates to human transsexualism, since the whole issue with human transsexualism is that the person is generally physically not intersexed or hermaphroditic, but behaves in a different way. So even the existence of hermaphroditic animals doesn't really seem to mean too much.
There might be animals that behave like the opposite sex despite not having physical characteristics of the opposite sex though, and I think that would be more meaningful.