Mob Morphing is a phenomenon in which an inhuman creature (for example, a Creeper) transforms into a humanoid state (like Creep). A few members of different species are able to do this, but it requires a mass amount of concentration, mental and physical capability, quick thinking, and, above all, self control. Being human is granting mass destructive power, while also changing strengths and weaknesses, and losing the ability to channel your fear as well. Also, there are some weapons you have to be roughly humanoid to use. Bows, swords and bombs are a few examples.
If something close enough to a human Morphs, it will retain traits from its original form.