Character IDs:

Center right is of course Tachyon. (The only non-OC of the group.)

Tachyon lookalike on center left with ear feathers is Agnes Flight. Artist also drawn her a few times (usually with her rival Shakur) with her being named in the commentary for those.

Mare sitting at center has flowers on her left ear, so most likely their mother Agnes Flora. (Could also be their grandmother Agnes Lady but less likely; she's usually drawn with Chinese clothes by various artists.)

The outer umas are harder to ID. That said, rightmost is likely Agnes Surgeon because of the medical cross ornament on her right ear.

Leftmost... well, if we're going with the theme then the umas at the back should all be Sunday Silence x Agnes Flora foals going from eldest to youngest. She also has a similar shade of hair as Flora and Surgeon, so she's likely a bay horse IRL. This makes Agnes Takao the most likely pick. I guess the bird ear ornament could then be a hawk to reference the Taka part of her name. (admittedly kinda a stretch, since most forms of Takaoo/Takaou aren't written with the hawk Taka.)