This is a recent craving of mine, so the whole thing would be pretty experimental for me. My ideal partner for this would be someone with some experience in RPs of this sort (eroticism/romanticism involving non-humanoid characters), who already knows that they like it.
The general idea would be that a man (played by me; specifications flexible) chances to have a lone encounter with a beautiful female of some exotic, non-humanoid (but intelligent) species, i.e. a unicorn or a dragon (played by you). Fear might turn to curiosity, to talking... to flirting, and then perhaps to things neither of them would have imagined themselves doing. The RP would probably take place in some sort of more-or-less generic fantasy setting.
To give some slightly more specific ideas, the thread could involve anything from a lonely woodsman bathing in a stream, spied upon by a curious, young unicorn, to a would-be dragon-slayer being seduced by the strange charms of his quarry; a young dragoness not nearly so massive and fearsome as he'd been lead to believe. Those are both examples of consensual scenarios, but if you'd prefer to do something non-consensual, then my character might be more along the lines of a perverse, rampaging demon or monster of some sort, scouring the land for an exotic mate with which to sate his dark hungers.
The exact nature of the female partner is up to you, and I'm open to suggestions. If you're interested, PM me with questions, plot cravings/ideas, or with your character concept.