Built-In Teamwork

One thing all three major super-teams at Marvel--the X-Men, the Fantastic Four, and the Avengers--have in common is that they've all fought a single being who's an amalgam of their powers and abilities. And it's happened relatively early in their respective runs. The FF were only a year and a half into their career before they were challenged by the Super-Skrull--who not only possessed their powers, but had them many times magnified. And being a Skrull, the alien also retained his shape-shifting abilities.

The Super-Skrull gave the FF a pretty good battle--but with such enhanced abilities, there came a time when his own Emperor felt he should be dealt with before he became a threat to the throne. And having created him, the Skrulls also had the foresight to create a defense against him:

Then there was the Adaptoid, a creation of Advanced Idea Mechanics whose goal was to destroy and replace Captain America. Cap, of course, is still around--but that didn't stop the Adaptoid from imprinting the abilities of Cap's fellow Avengers on himself and thereby becoming the Super-Adaptoid:

And in the ensuing battle, he thought he'd done away with Cap. But, realizing his mistake, he made more attempts to destroy his target--first by facing the entire Avengers team, where he actually had them on the ropes as he now possessed the abilities of even their most powerful members.  But he finally went down due to, of all things, a design flaw:

Then, taking a more subtle approach, he later decided to lie in wait for Cap at Avengers Mansion.  But he got a little more than he bargained for when Iron Man showed up instead:

In the case of the X-Men, their foe turns out to be an embittered man named Cal Rankin, who also goes by the rather appropriate name of the Mimic:

Rankin is something of a precursor to Rogue--only it's his proximity to others that allows him to mimic their powers, rather than needing to come into direct contact with them. And there's one more important difference: where Rogue fears prolonged contact which risks her borrowed powers becoming permanent, Rankin concocts a ruse to join the team in order to mimic their abilities and reach a machine which he hopes will do just that. And when he makes his move, it's indeed an impressive display:

What Rankin didn't know was that the machine was rigged to remove his mimicking power, and thus his threat was ended, at least for the time being.

I suppose if the original Defenders had been together long enough, they would have faced their own amalgam.  And then there are the Sentinels, who are somewhat like the Adaptoid in function but with more emphasis on creating counter-measures.  I don't recall any other examples--anyone care to chime in?