Squid (Men in Black)

"Congratulations, Reg. It's a... squid"

- Agent K on the birth of Reg's child

Reg's species is an unnamed race of sapient, cephalopod-like alien beings, some of which are known to live covertly on Earth, possibly as galactic refugees. Only their infants have been seen undisguised and judging from their appearance this is a slimy brown-skinned species with large black-eyes, semi-humanoid anatomy, multiple tentacles and, oddly, a mammalian-style navel.

Apparently, giving birth can be a violent experience for this species. In the case of Reg's wife at least, she extended a huge tentacle from her body and proceeded to attack Agent J, who was attempting to help her with the delivery.

Behind the scenes
It is possible that this is the same species as the Cephalopoid seen in the beginning of the movie, whose true appearance is unknown but highly likely to be squid-like based on their name. When Reg's baby is seen blinking it does have a pair of inner eyelids, but these are highly different from the Cephalopoid's gill-eyelids, making it more likely that they represent different species.