I use different methods for my particular games, depending on my mood.
Method #1: No Real Reason
Giant Horse - "Hunter" (because I use him mostly to run-down animals to get gourmet raw meat to sell
White Stalion - "Sprinter"
4/4/5 All-Black Horse w/ "Ancient" horse gear - "Shadow"
4/4/5 "Link's Fake Horse" - "Courage"
4/4/5 "Fake Epona" - "Epona"
Method #2: Based Off of Horses from Mary Stanton's Book, Heavenly Horse of the Outermost West
Giant Horse - "Anor"
White Stallion - "Equus"
4/4/5 All-Black Horse w/ "Ancient" horse gear - "El-Arat"
4/4/5 "Link's Fake Horse" - "Duchess"
4/4/5 "Fake Epona" - Epona
If you wonder what I mean by "Fake Epona," You can get a better idea in my recent post (and unofficial poll),
HERE on these very forums.