Like Happy Meek, Bitter Glasse is an "original umagirl", i.e. not based on any particular real-life horse. However, as it is often the case with popular pop culture, there's a real-life horse named after her.
Bitter Glace (note the different spelling) is a bay filly (not a colt as one would expect from the ear decoration placement) sired by Rulership and foaled by Anthology. Rulership is a son of King Kamehameha and Air Groove while Anthology is a daughter of Sakura Bakushin O and Pops (herself a daughter of Sunday Silence).
As an apparent nod to her, Bitter Glasse is said to go to the Miho dormitory according to dorm head Hishi Amazon.