Baby Cthulhu is the baby form of Cthulhu. As Cthulhu is ageless, Baby Cthulhu makes absolutely no sense at all but sense isn't really a prerequisite for having a baby form, and Baby Cthulhu is beyond the mortal comprehension of the word anyway.
Baby Cthulhu is said to look like a monster of vaguely anthropoid outline, but with an octopus-like head whose face was a mass of feelers, a scaly, rubbery-looking body, prodigious claws on hind and fore feet, and long, narrow wings behind. He also wears a nappy.
Baby Cthulhu is capable of destroying the entire world, and his hideous appearance is enough to drive people to insanity from the mere sight of him. He enjoys murdering millions, destroying entire civilizations and long walks on the beach.