Master Yen Sid is a human being.
Unspeakably old and wise, Yen Sid is a very powerful and knowledgeable wizard. He lives in his castle (the exact location of which is unknown), where he spends his days experimenting on his magic powers. Yen Sid gains much of his magic powers from his hat, though he also has abilities of his own. Mickey Mouse has been Yen Sid's apprentice in magic and protégé since 1940, though Mickey's recklessness and mischief have made his training very slow.
Behind the scenes
Yen Sid first appeared in 1940 in the Sorcerer's Apprentice segment of Fantasia.
The name "Yen Sid" is "Disney" spelled backwards with an added space; this was an in-joke from the animators during production of the original sequence, as they'd actually modeled some of Yen Sid's facial expressions on Walt Disney's own.
However, knowledge of it remained uncommon for several decades; in the 1971 TV special Disney on Parade, in which he was prominently featured, once again monitoring Mickey in his magical tricks, he referred to himself as "Merlin the Great" (even though several other magicians called Merlin already exist in Disney lore, two of which have appeared in the comics universe). In all of his appearances in comics, he likewise remains unnamed. His name was only revealed to the public with the video game Kingdom Hearts II in 2005, and has since been accepted in non-Kingdom Hearts-related media.