Season 3 (Prequel)
From Glitchtale Wiki
Season 3 of Glitchtale (more commonly referred to as The Prequel) occurs in the same world, but is situated 1,000 years before present day, and will elaborate in detail the events leading up to the War of Humans and Monsters mentioned in the beginning of Undertale.
Prequel Shorts - Glitchtale Origins
Glitchtale Origins: The Black Beast
Glitchtale Origins: Kanashi Fukitsu
Glitchtale Origins: Kody Fallenfire