I've confirmed this seems to only happen to any mount above the regular Hog. Level 1-3 hog cannot replicate the glitch, but level 3+ silver mane and draco can. Basically, every time you use a Revitalizer, your exp increases and your mount levels. Even after it's at 0 tiredness. Also tiredness seems to go down relatively fast on Silver mane compared to Hog. But that I'm not 100% sure about yet.
Tested on Draco. Levelling system doesn't seem to be working correctly. I could easily level it a few times by consuming a few revitalizer.
