One of MiB's major villains is, ironically enough, one of it's founding members, Alpha. Alpha went corrupt upon finding the "Cosmic Integrator," a forbidden device used to fuse things together, and immediately saw that he could use it to make himself far more powerful by grafting alien parts onto his person.
In his first appearance ("The Alpha Syndrome"), Alpha attempts to steal a Sintillian's heart (they normally have two). The heart is nigh-immortal, and appears to have a will of it's own.
The Sintillian itself:
Alpha fails to capture either heart and is caught in an explosion, seemingly killing him.
Alpha's next appearance is technically "The Quick Clone Syndrome", though chronologically it makes more sense for it to be "The Heads You Lose Syndrome", as we'll see in a minute.
In the "The Heads You Lose Syndrome", Alpha is in need of 'replacement parts.'
At first he hires Jeebs to literally take limbs off unsuspecting aliens, but then finds out about a Symbiote species that has regenerative properties. This plan backfires as the Symbiote heals all the grafted alien parts, too.
Needless to say, Alpha has to go hunting for new alien body parts himself.
Sometime before "The Quick Clone Syndrome", Alpha cocoons himself in the sewers, presumably to undergo some kind of caterpillar-like metamorphosis.
The Alpha that emerges is quite bizarre:
He retains this body for the next couple of episodes he appears in, and occasionally exhibits a few strange features.
Within the folds of his back (or perhaps a cloak) is a curious second head:
He is able to spawn at least one mini-clone-thing:
And, while out in open space, grows a gas-mask of sorts and a pair of antennae to "speak" telepathically:
Alpha eventually falls into the ocean near Antarctica and suffers severe frostbite and once again goes off to claim more replacement parts. This time he takes them from several alien convicts in a small prison-ship that happens to be traveling near Earth as well as one very unfortunate ice fisherman.
Agent J causes this new body to essentially self-destruct, which reveals that Alpha's original human body was still relatively intact at it's core though severely weakened. MiB locks up Alpha while keeping him on all sorts of life-support systems.
As one would expect, Alpha eventually escapes (using nothing but parts from an electronic chess game) and fashions himself a new body, but this time he uses machinery (having noted how much alien parts have failed him and how machines had kept him alive while incarcerated).
Alpha is finally killed in this form thanks to a missile that would have destroyed the Earth.