The non-human aspect is usually outside my experience but recently I've found myself trying a number of new things and roles so perhaps I could take you up on this one? I'd be delighted to play the non-human creature and we could either discuss appearance and size over a series of images or cook up a creature between us that has the features we want.
I. too, amd curious about knotting and barbed cocks although by 'barbed' I would consider semi-flexible fleshy spikes or nodules. Something to tease and stimulate the hole it is thrust into, more than tear it or inflict pain or fix the cock within the body (though we can have that aspect too if you want).
I am seeing a much larger creature than a human male, perhaps something eight to nine feet tall with the sexiual organs to match so that anal intercourse with teh human will be either agony or would take a long time to train/stretch the male.
A long reptilian tongue is a possibility too, as is a small tentacle appendage that could be inserted into the human male urethra to stimulate his cock intenally. It depends how weird you want to go and what you want from the RP.
This also sounds to me like the human would be taking a more submissive role.
What setting had you in mind? I would quite like to go for a fantasy world much like ours and contemporary but with an alien race that has been on earth for some decades and has integrated into society. Are you more seeking a one-shot here or a story about a relationship?
Would impregnation be on your list of interests?