I've asked this question before in GP Forum when I first began the game, no one has a very pleasing answer other than Google symbol pages and trial and error until one works. Unfortunately, there isn't a magical place to go with a list of all valid fonts and symbols. There used to be, but most of them no longer work. Apparently, Howrse cracked down on fonts and symbols in last couple years.The best way, is to copy/paste symbols you come across on Howrse via horse names, team names, and affixes. Put them all in your notebook and you can come back to them when you are looking to spruce up a horse name.

BB