It’s not only the horses , all pets inventory work this way , I beleive for Quality of Life purpose since they know from at least 2018 and with all the changes that the pet hunger system have had :
- From having to be fed in order to keep em alive ( that applied to thralls also )
- to the food put in their inventory was insta - consumed , and the pet/thrall would get the buff for the amount you put in , that added both the buffs the food gave to pet/thralls , and the pork bug situation
- to the leveling system where food now stays shown in inventory , provides heal or buff AND the addittional % bonus to one or more of the pets/thralls stats
yet they never aknowledged this particular behaviour was unintended !
So people ( and me ) just presume it is Working as intended.