Zombie horsemen don’t seem to appear in meadow biomes. As plains villages spawn in meadows, it would be fair to assume that they’re a plains variant, though this could be intended due to the hillier terrain and smaller patches of this biome.
MC-303014
Zombie horses don't spawn in meadow biomes
Open
1
Can comfirm using the MC-302818 setup but with a single-biome plains world. For what it’s worth, I don’t really consider meadows a plains variant since they’re alpine in nature.