New disguiser mob

This would be a pretty rare, hostile mob with low health, and above average damage. It would (as the name suggests) disguise itself as a random mob. The disguiser wouldn't look exactly like the mob it would disguise as, think of the tint being a little off, It being a little smaller/bigger, or even making the wrong sound! Be creative! Whenever you look at the disguiser, it would act like a passive mob (even when he is disguised as an hostile one!). And when you look away it would slowly approach you to attack. Whenever the disguiser gets damaged he would take off his disguise and run till out off sight, when out off sight he would take on a new disguise, and try to attack you again.

P.S: Sorry if there are loads of misstakes in this text, im Dutch and thus not a native speaker.