Mob: The mimic

This was originally a part of an April fools suggestion on discord, but I’m going to make it a standalone suggestion vs with cosmetics like originally.

A mob that has a chance to spawn from chests (I was thinking only in witch huts, but dungeons work too.

When active, the mimic has an Ai range... kind of like what the zombie has, to notice the player from far away... but it uses its' range to notice other hostile mobs that have spawned close by and in a puff of smoke transforms into that mob to attack the player. Say it transforms into a zombie, it'll either have 2x the attack or 2x the defense.
Example: Maybe 2x defense so takes longer to kill but weaker attack then the real mob? (or vice versa)

To tame the mob you'd 'feed' it a few blocks of a rare gems/ore (diamond, emerald, plat,etc) It won't attack the player (or other mobs) after tamed, but can still transform into mobs close, now included non-hostile mobs.
After a while it'll go back into mimic form due to a cool down, then change to another/same mob if one is close by. To keep the mimic to an area (without the need of a lead) place down a rare ore block and it'll stick close to that block. Hopefully you won't kill them by mistake.
This is a way to have wandering mobs but have them sticking to an area you want them to stay in (like how tuff golems work). No more wandering clear across the other side of your realm, without the need of leads or trapping the mob you want in a specific area. Just hide the mob you want them to turn into below the gem/ore block, so most of the time they'll turn into that mob. (If you want them to stay a mimic have a chest placed below their gem/ore block) .

Another option would be the mob in an item framed capture net, since if you want a wandering harmless warden for example, can’t really have that chilling under a block. Unless A new drop item from all mobs, similar to a mob head could be placed on/under the block the mimic could then transform into.

Don't know if this mob is possible to be made, but thought it be a neat mob for players that want more life/variants of life in their realms.