The textures of the horse (llama_armor_slot, armor_slot, saddle_slot) and loom slots (banner_slot, dye_slot, pattern_slot) are located in the ".../textures/gui/sprites/container/horse" and ".../textures/gui/sprites/container/loom" folders, which is inconsistent with other slots - for example, slots of the enchanting table and the smithing table (empty_slot_lapis_lazuli, empty_slot_sword, etc.) the textures of which are located in the ".../textures/item" folder. 

This issue should be fixed due to unexpected behavior when creating resource packages and other types of modifications.

A possible solution of the issue:
Renaming the above texture files to empty_slot_carpet, empty_slot_horse_armor, empty_slot_saddle, empty_slot_banner, empty_slot_dye, empty_slot_pattern respectively, and moving them to ".../textures/item" folder.