Doppelganger Mobs

This idea may be similar to others, but I think this would be a great feature and a fantastic pastime to do for any player. My idea is to add mob called Doppelgangers (or Demons) that look exactly like a given mob, disguising themselves. The difference is they have a ton more health, and that's how you can tell them apart. If you want, they could have slightly higher attack damage as well. Doppelgangers would be very rare, so players will have to search for them. Depending on what mob they are disguised as, they should match the exact same spawning patterns. Doppelgangers could also disguise as passive mobs, where they would be non-aggressive until attacked, like a neutral mob. A really cool reward for defeating these mobs could be their mob head. That would be a good reason to go ahead and add many other mob heads. It also provides another way to get mob heads without the risky Charged Creeper. However, they could drop a completely new item or maybe just drop way more of the normal mob drop. I really hope this idea gets seen. Thanks to anyone who read!