The Leaftaur King, also known as the King of the Leaves, is a minor character in Centaurworld and is voiced by Chris Diamantopoulos. He appears twice in the show, first in episode 5 of season 2 (Bunch O'Scrunch) and again in episode 8 (The Last Lullaby) of the same season. In both appearances, it is his birthday.
The only other online mentions I could find of this character are currently on his voice actor's IMDB credits page and my sister's Tumblr where she posted pixel art I commissioned her to draw of him. Seriously, he's not even on the wiki! It is thus my noble goal to prove that he indeed exists and that it is once again his birthday.