Carmilla and all her more-or-less human entourage, in the Whateley Universe. In fact, with a major-league lust demon for a father and a human/Deep One hybrid mother, Carmilla is also the result of one of those...and from what we've seen of her dad since he showed up in the stories, which basically paint him as a devilishly charming rake with some trouble keeping his tentacles to himself who honestly dotes on his only daughter, romance may well have actually figured into it.
Aside from the above, we've also seen hints at possible romance between one or two of the students and members of the local tribe of lycanthropes, and of course there are plenty of mutants whose mutations have forced them into more or less inhuman forms with little hope of reprieve. (The technical term is GSD, for 'gross structural dystrophy'.) While the latter may still be technically and legally human, speculations about their potential present or future love-life (such as the Fury Twins idly musing on their chances of finding a boyfriend) necessarily do invoke this trope.
The art of Fredrick K. T. Anderson does this a lot, with plenty of hybrids, half-human or otherwise, resulting. The Bard is the most infamous, having bred with a dragon, a werewolf, a minotaur, a naga, a scorpion woman, a centaur, and a rock. The last case is lampshaded.
Neopets, full stop. It seems that any species of Neopet can breed with any other, leading to what can best be described as 'unusual' family combinations...To the point where a simple family of four could all be different species.
The Nostalgia Chick talked about how this was a bad idea in her "Science going wrong" video. "Don't put your dick in it!"
But she also says “beauty soothes the savage beast,” if you’re female, you should let the creature put its dick in you.
In Disney World of War there are several and the most prominant are Scatani, Scamp (dog) and Vitani (lioness) and Berlielle, Berlioz (cat) and Danielle (dog).
Someone keeping track from Elcenia's Rhysel's point of view would assume this was the norm on Elcenia. Though it isn't entirely, it is assumed that a dragon will go through a number of non-dragon mates before settling down with a dragon—dragons live a long time.
The Journal Entries are practically MADE of this trope. There are VERY few ongoing relationships in which all people are the same species.
Has happened sometimes in We Are All Pokémon Trainers. At some point there are romances between humans and Legendaries, such as Luke with Reshiram and Jane with Zekrom. In a later case, Sierra's boyfriend Vyvyan is baleful polymorph'd into a Rayquaza.
The website of the Neocene Project includes a creature that is the result of hybridization of coyotes, domestic dogs, and jackals (hybridization between three species?)
Foul Bachelorette Frog is a red-eyed tree frog, and Foul Bachelor Frog is some type of pond frog. But the two memes have been combined into "Foul Frog Couple."
Enoriel is revealed in later episodes of Reflets d\'Acide to have had a romance with Alia-Aenor, though she was posing in a human form when they met. As soon as he learnt what specie she really was, he ran away out of fear.
