Albane's Extra Textures is an addon for the mod Realistic Horse Genetics. This mod adds 63 new face markings that can appear along side the 6 in the base mod.
This mod is client side only and must be loaded with RHG in order to run. Joining an existing world with this mod will have all horses change their face textures. Uninstalling the mod will have all horses revert back to default face textures. Horses will get assigned their textures based on their genetic values just as in the base mod-however there is a config option to disable this and have horses get randomly assigned face textures instead.

Check images tab for a showcase of a small selection of them

This mod also has a framework to allow you to add any face textures you want to horses!

To add more textures, first create a resourcepack that contains the new textures in the location

PACK_NAME\assets\horse_colors\textures\entity\horse\face

Then go to the configs. There you will find 3 values, a list of all possible face textures, how many face textures, and what textures are allowed with non splash.

Simply add your texture's name (and/or remove others) in the correct ordering you wish for it to appear. Set the correct numerical values with the adjusted list and load up the game!

Math is not my strong suit, so the way I'm calculating which texture to choose is probably not the best-if you are good at math and have a better way of doing so, please share lol

Credits

OutrightWings - Programmer
Albane_mcpe - Artist
Sekelsta - Developer of RHG and sorted the textures by white value