The Maestro is or was a human man.
Description
A mysterious, highly esteemed artist from 1930's Belgium, the Maestro was the owner of a “Liquor of Life“ which granted full life to whatever he drew if he sprayed the drawing with it — not just as Toons, but as fully flesh-and-blood beings. In 1938, at the request of M. Papillon, the Maestro created the perfect bellhop — mischievous and resourceful Spirou.
Behind the scenes
The Maestro appears in the 1938 comic page La Naissance de Spirou — The Birth of Spirou in English. Being a Spirou character, he does not belong to the Walt Disney Company, although Spirou is considered canon on this wiki.
Identity
Rob-Vel's 1975 cartoon where he draws what is explicitly himself to resemble the Maestro.
The real-life, accepted creator of Spirou is Rob-Vel, who drew his serialized adventures from his inception in 1938 to 1943 (with occasional reprisals).
It is thus commonly reckoned that the Maestro is Rob-Vel. However, he is actually a caricature of Rob-Vel's friend Luc Lafnet, who occasionally helped him with Spirou and other creations of his, off the books.
Moreover, casting doubt on the Maestro's in-universe identity is the fact that Rob-Vel would later caricature himself in a way that heavily resembles the Maestro, except with Rob-Vel's mustache replacing Lafnet's beard.