A third M.U.T.O. was introduced in Godzilla: King of the Monsters, but this Titan isn't like the other two creatures previously seen in the MonsterVerse. The new M.U.T.O. is one of 17 Titans confirmed to exist in Legendary's MonsterVerse. A handful of the Titans are comprised of classic Toho monsters, but the rest - including the third M.U.T.O. - were created specifically for movies set in the MonsterVerse.
In Godzilla: King of the Monsters, the actions of eco-terrorists set the stage for the Titans of the MonsterVerse to awaken after centuries of inactivity. Emma (Vera Farmiga) and Jonah (Charles Dance) make the mistake of waking up the alien Titan, King Ghidorah, a three-headed monster who defies the natural order and summons all the other Titans. Godzilla, Rodan, and Mothra are all drawn into the conflict, with Godzilla and Mothra on one side and Rodan on the side of King Ghidorah. After Mothra sacrifices herself, Godzilla manages to defeat Ghidorah. Following Ghidorah's death, Rodan and the other Titans bow down to Godzilla, thus accepting him as the new King of the Monsters.
Joining Rodan and Godzilla in this final scene are four new Titans. Three are completely original, but one is of a species that's already been introduced to the MonsterVerse. The creature, which has no official name, is a third M.U.T.O. (Massive Unidentified Terrestrial Organism). The M.U.T.O.s debuted in 2014's Godzilla. A pair of M.U.T.O.s were the main antagonists of the movie and the first enemies ever encountered on-screen by the MonsterVerse's Godzilla. But while the third M.U.T.O. is similar to the previous two that appeared, there are some key differences, which may suggest that this version is perhaps of a subspecies of whatever species the M.U.T.O.s are in Legendary's franchise.
Apparently, the M.U.T.O.s are old foes of Godzilla, which is why he takes it upon himself to hunt them down throughout the movie. After a brutal fight, Godzilla succeeds in killing both of them, but apparently their species lives on. Fortunately for the third M.U.T.O., Godzilla has decided to let this one live, possibly because it's now under his control, or because there isn't any danger of it reproducing.
This M.U.T.O. is female. It was established in Godzilla that female M.U.T.O.s are substantially larger than males, have eight legs, and can't fly. These attributes all seem to apply to the Titan from King of the Monsters, though there are a few minor physical differences which could suggest that the third M.U.T.O. is of a slightly different species. For instance, the female M.U.T.O. from Godzilla is larger than the one that appears in King of the Monsters. Also, the third M.U.T.O. has three fin-like structures on her back, which the other M.U.T.O. lacks. The third M.U.T.O. is living proof that Godzilla hasn't eradicated them all, and it remains to be seen if more M.U.T.O.s exist. After all, it's been suggested that other Titans are hidden somewhere in the MonsterVerse.