The first alien shapeshifters introduced in the Ultimate Marvel Universe are known on Earth as the Chitauri, who financed the Nazi regime in an attempt to conquer the Earth. Their leader Herr Kleiser states that they are called by other names on various worlds, including the name "Skrull". These Chitauri claim to originate in the "lower fourth dimension", and assume others' forms by devouring them; it is unknown what their true form looks like, since the only time one of them is shown in it is after it has been killed and dissected. The Chitauri have a hivelike social order, with the "officer caste" having superhuman strength, durability and high intelligence. They are defeated by the Ultimates.
Another race called Skrulls, physically resembling the Skrulls of the mainstream Marvel universe appear later (led by the billion year old Skrull Emperor); these Skrulls dislike being confused with the Chitauri, whom they call terrorists. They have extremely advanced technology, but have not been observed to shapeshift. These Skrulls were seen only in an alternate timeline in which Reed Richards contacted their world via his teleporter; the events leading to that timeline were altered in Ultimate Fantastic Four #29 so that contact was never made.
To develop their conscience and consciousness, to make them aware of what is going on, to prepare the precarious ground for the future alternatives—this is our task: "our" not only as Marxists but as intellectuals, and that means all those who are still free and able to think by themselves and against indoctrination, communist as well as anticommunist.