Wyverns

10 HP at level 20? That can’t be right, it’s nonsense. Who would need that? Here’s a quote from the manual for version 3.0:

Wyvern Tamer
Wyverns, dark wyverns, and pao-kai in the hero’s army gain +2
hit points for each hero level, starting from the first.

20 x 2 = 40, as Vortex already said.