Satyr, a nature spirit from Greek mythology.

Fun fact: Wikipedia says they have a "permanent, exaggerated erection". So that seems very appropriate for this site.

Haven't tested in too much variety, but using any style of horns should work well. I've distinguished horns and antlers in the tags.

Use monster boy or monster girl, and anthro/furry if you want a more "beast" look rather than "human" look.