Irayo was born to a fawn woman (like a satyr but in both genders, rather simple minded), his father was an Incubus. He was a fawn until he came of age and went through the changes making him a demon and giving him his cubi powers. He is not a ravenous predator, but he cannot help himself when it comes to flirting. Otherwise he is pretty dorky.
Our cubi feed mainly on emotions, typically lust, and through dreams. They can also change their form, though it takes them hours, except for a select few forms that they have practiced enough to be swift at. One usually is their original species. Irayo can become a fawn and a barn owl.
The birth mark on his tail is a mark of his father who is an ancient demon and so has the power to pass that sort of thing on.