Here is my theory. The Muton we saw on Xcom 1 and 2 were pumped full of drugs which generated the look we knew. The Muton we see on Xcom Chimera Squad is what they look like without the Elders meddling.

Talking about meddling, here is my theory for Chimera Squad, it's all conjecture but I'm sure it will happen so you may as well treat it as spoilers.

Chimera Squad will do a bunch of SWAT-like missions like bomb defusal and hostage rescue.

There will be at least one scene of human prejudice toward aliens JUST after they save the day.

Aliens will be paramount to solving some problem, like Sneko blood being used to cure some nasty disease.

One of the humans will betray Chimera Squad, probably the beard guy.

One alien will sacrifice himself for the greater good. Probably the pink one.