This extremely large alien only appears in the final scene of Men in Black, when the camera zooms out it is shown an Alien playing with a marble, which has the reflection of Galaxies, the one in particular is the Milky Way. the same being true for several other galaxies as well. The alien, which has a greenish gelatinous appearance and four digits on each hand, is seen playing with some of the marbles before packing them on a bag.
This is likely to be the second-largest alien featured in Alien Species Wiki; only being outcompeted by Yog-Sothoth.
Note: Look at the reflection on the picture.