Don't confuse these with a set's SEC (Secret Rare) chase cards, though: those are usually separate Character-type printings of a popular character, not alternate versions of that character's Leader card
This mechanic is remarkably similar to the Crew mechanic introduced in Kaladesh but unlike vehicles that need to be crewed, mounts can attack and block as normal, however by saddling them they activate unique and powerful abilities
Allowing your creatures to deal damage off toughness rather than power is great with defenders and other creatures with large butts, like Giant Ox
The sets from the last few years are legal in Standard
Whenever another permanent you control is put into a graveyard from the battlefield, if it had one or more counters on it, you may put that number of +1/+1 counters on target creature
But so many powerful cards are noncreatures