I don’t think there’s a cutie mark on any of the things he’s in, but I decided it’s a dark gray/black arrow.
Images:
Sleeping foal-
Forum/viewtopic.php?t=2284650
Adult-
Forum/viewtopic.php?t=2289494
Adult-
Forum/viewtopic.php?f=32&t=2517741
I hope his design isn’t too simple…You could do his father or his niece instead but I’d have to get you a reference for her.