Add tag to biome; then spawn X mobs in biome with tag

Like the title says; is it possible to simply add a tag to a specific biome (tag:worldgen/biome:mydatapack:more_horses to biome:regions_unexplored:redwoods), and then make a specific mob spawn in a biome with the tag I've created?
All help is appreciated. I've been trying to look around for how to do this, but can't find how it's done without relying on mods. I'm very new to datapacks but I'm trying my best!