Gus's parents are purely responsible for his batlike appearance, despite him being entirely human at the core; Magical body modification is controversial and dangerous enough as it is in the modern day of MM, but when Gus was a kid? Even worse.

They wanted to join a fad among the rich and well-to-do of the fifties of making (often neurodivergent or just odd/misbehaved) children more "cute" and acceptable with magically applied animal features. Their goal was a catboy, but due to the unpredictable nature of magic and a not particularly skilled wizard things went sideways, resulting in weird dormant magic that led to him sprouting bat features as he aged and scrambling his brains a little bit.

Regardless, he's happy as he is now. Definitely not for the best but after all that hard work, how could they not be proud?