The intergalactic musclehead known as the Muton will be back for XCOM 2, but he's not quite the same as he was in XCOM: Enemy Unknown. The hulking slab of goonishness has been upgraded with a dose of human genes, adding "the cunning intellect of the human mind" to his devastatingly powerful physique. This, obviously, is not good for us.
XCOM 2 Mutons appear to maintain most, if not all, of the same abilities they had in the preceding game. They can take advantage of the Suppression ability, significantly lowering the aim of their target while allowing a free reaction shot if the target moves, and can toss grenades, which are bad news for tight groups of soldiers.
But their up-close nastiness is enhanced by bayonet-equipped weapons and the Counterattack ability that gives them a chance to ignore all damage caused by a melee attack, and launch an immediate melee attack of their own in response.
The new Mutons look a bit different as well: Meatier, believe it or not, with a whole lot more neck and less dependence on the bulky exo-suit they wore in Enemy Unknown. They actually look rather Krogan-like now, which I would take as another sign that you should govern yourselves accordingly if you run into one of these guys in a dark alley.
XCOM 2 will be out in November.