348px-FO3 Yao Guai concept art
Whether its Zion Canyon or the Capital Waste, Yao Guai can be a huge threat. As mutated (possibly goulified) black bears they are fast, strong and more than willing to make you into their next meal.

Because of the way that they attack, they make for good practice when training to take on Deathclaws. Having the perk Animal Friend keeps them from attacking unprovoked.