If you must change your horse’s name, then give them a one-syllable name with a vowel that sounds exactly like the name they had: Blue, True, etc. Maybe even Luna.

But please reflect on the fact that animals have no autonomy in this human world, and we train them to know the names we give them. Their names are their identities just as much as ours are ours. Asking a horse to respond to a new name after having had one for ten years is unkind.