Yao Guai are a form of Mutated Bear in Fallout 3. If you encounter one of these beasts in the Capital Wasteland, you had better prepare for a fight... and prepare fast! Yao Guai are strong, durable creatures that have a lethal slashing attack, but the biggest threat is their speed, they are one of the fastest creatures in the game along with Deathclaws and you'll have no chance of out-running them. They also have a high perception skill, meaning they are more likley to spot you from a far distance.
Yao Guai are one of the best reasons to get the Animal Friend perk, this will stop them from attacking you. Obtaining the second rank of the perk make them come to your aid if they see you're in trouble.