The Giant Baby is a boss faced in the video-game "Zombies Ate My Neighbors" - appearing (unsurprisingly) as a gigantic toddler this boss is most likely a result of the many mad experiments of the game's main antagonist Dr. Tongue, though how and why this particular baby grew so large is left to the player's imagination as the game didn't dwelve far into plot (being a parody/homage of many horror cliches).
As a boss the Giant Baby can be difficult or easy depending on your relative skill and caution. Once defeated, the Giant Baby shrinks down to normal size and can be rescued just like the other "victims" in the games.
The Giant Baby is faced again in a Bonus Level named "Son Of Dr. Tongue" - though it is likely this was a different baby, though they both attack the same and share the same appearance (the appearance of the Giant Baby in this Bonus Level, coupled with the level's name, seems to add backing to the previous theory on Dr. Tongue being behind the unusual appearance of giant toddlers).
Yet another Giant Baby is faced in the level, "Look Who's Shopping", (an obvious parody of "Look Who's Talking"), where it is inexplicably terrorising a local shopping mall - much like the other two Giant Babies this titanic toddler can be dangerous to players regardless of their experience due to its surprising speed for its size.