- "Brian: Look it has blue eyes?
ALF: Naturally, it is a cockroach."
- ―Brian Tanner and Gordon "ALF" Shumway
The Melmacian Cockroach is a blue-eyed insect commonly found on planet Melmac. Although naturally tiny, they can grow to the size of a Human, or even bigger, when exposed to certain earthly chemicals, including boric acid.
After Melmac was destroyed, a Melmacian Roach, named Rodney by ALF and Brian, escaped to Earth in ALF's spaceship (in a bag containing a slimeball) and became huge and aggressive after the Tanners' attempts to kill it. Eventually, ALF discovers a substance that is lethal to the Cockroach: perfume.
Since Melmac no longer exists, it's possible that Rodney was the last member of his species, meaning the Melmacian Cockroaches are now extinct.
It is unknown what the cockroach would have done to ALF if ALF had lost the battle in the bathroom. It does seem that the Cockroach chases ALF throughout the house, as the cockroach had perfect following steps to find where ALF went.