Thanks for the idea! We checked how the original game handles feeding a mount. Revitalizers lower the mount's tiredness quietly, with no chat message. The way to check is the TAMING MOB page in your Character Info window, which shows the tiredness bar. A chat line like the pets have would be a new feature rather than a fix, so I've passed it to Mark to decide and left this open until he does.

Update: Mark liked the idea, so it's in now. When you feed your mount a Revitalizer, a line pops up the same way it does for pets, saying how tired it still is, e.g. "Your mount munches happily. Tiredness: 40/99". If it's fully rested it tells you so, and if it wasn't tired at all you'll know not to waste another one. Pets got the same treatment: their feeding line now ends with how full they are, e.g. "Fullness: 70/100". To re-test: ride your hog until it gets a bit tired, feed it a Revitalizer, then feed your pet some Pet Food.