Do all mounts Count toward lord and hero % I believe they do, and that makes sense. If they do, then a skink priest on an ancient stegadon would count all the percentage points toward Heroes. My question is, does that ancient stegadon also count toward the total number of rares you can have in your army?
Mounts only count toward the hero or lord allotment of their rider as they represent a unit "upgrade". They do not count toward special or rare point allocations or restrictions on duplicates. Previously, this was the case but the rules were changed in 8th edition when composition went to a % / category approach.