The horse on the left was patiently waiting its turn until the horse in front of it had finished eating and walked away. Only then did this horse approach the tree and start eating the bark. The bite marks on its back also show it is lower in the pecking order
The whole ceremony and the horse’s expression remind me a bit of those Early Netherlandish altarpieces in which sometimes the (very rich) donor is also depicted, humbly on the side panel.
The tree they are eating the bark from is probably a black alder (Alnus glutinosa), judging from the orange discoloration after the stripping of the bark.