These cryptic and ugly things are actually very rare. There weren't very many horses to begin with and after the nuclear war, most of them died out.
After some adventurers ventured into the forest, they saw these skeletal creatures roaming the area. They were very calm, but once they were spotted, the Skeltarse became very aggressive and attacked the two people.
Threat level: 50%
If killed by one, you will live the rest of your afterlife as a skeleton or zombie riding one of these.
They can be tamed.
They are strong when attacking, but have low defense.
They only come out at night.
Their eyes glow green due to the radioactive waste that powers them.
There is all ways a larger bulkier leader in each herd.