Found a strange issue where horse armor doesn't completely render if the mod is installed alongside Iris (no shaders need to be enabled).
Disabling either Iris or Horseman fixes this.

Tested on both NeoForge and Fabric (1.21.1 versions, with only the necessary mods and dependencies installed)

When both are installed:

Strangely enough, it renders fine in the inventory thing:

How it looks when either Horseman or Iris is disabled (how it should look):

Log from when both were enabled:
https://mclo.gs/yu5n189
Not sure if the log provides anything helpful.