A skrasher.

A skrasher is the adult form of the Alien Spawn illness which is caught by petting infected memau. They are big, black, chitinous monstrosities.

A single skrasher embryo will incubate inside a hapless victim before hatching explosively as a full adult of unbelievable size and begins rampaging through the station, destroying everything in its path. Its terrible claws can instantly rip anything it comes across to shreds, killing peeps, destroying buildings, and making a mess of the entire station. It takes a veritable army to bring one of these things down.

The only way to avoid them is to make sure that those infected with the spawn larvae have quick access to skilled medical assistance. Only then can the parasite be neutralized and removed from the host before it turns into... THIS.

If a skrasher manages to rage long enough without being killed it will attempt to find a spot without anyone to attack and start to shrink in size.  Left uninterrupted it will eventually turn into a memau.