"So it IS true that there's a monster that eats naughty kids..."
An unused line of text in Super Mario 64 DS references a "monster that eats naughty kids". This was likely unused dialogue from Tuxie in Cool, Cool Mountain when eaten by Yoshi. However, if the Personalization A.I. discovers the line of text, it may reinterpret it in some fundamental way. The resulting anomaly is a bizarre creature some have said to roam Cool, Cool Mountain in the Tuxie mission.
Description[]
The Child-Eating Monster has been described as several things, likely due to it being a product of the A.I. Some say it's an edited Bowser model, others say it's a large Scuttlebug, and yet others say it is a completely unique model. It typically only appears during the mission where you have to bring Tuxie to the mother penguin, attempting to chase you down and steal Tuxie. From there, they'd run off with them, usually requiring you to chase and attack them to make them drop Tuxie or restart the mission if they escape.
Alternatively, it can appear in a unique mission where it stole Tuxie from the mama penguin and you have to fight it in a boss battle. Where you fight them is usually inconsistent, but it is usually on an addition to the map or even an anomalous sub-area similar to Cold, Cold Crevasse. The battle itself varies heavily due to it's appearance also varying and also due to it being created entirely by the A.I. However, the fight usually involves some kind of biting attack, likely due to it's status as a child-eating monster. Upon defeating it, it will either drop Tuxie and Power Star, or the player will have to return Tuxie to the mama penguin to clear the mission properly.
Theorizing[]
Due to the variation in manifestations of this anomaly and it's correlation with an unused line of text, it is believed the Personalization A.I. created this anomaly with the minimal data of a single unused line of text. The anomaly itself is likely not meant to be as disturbing and potentially scary to children as it appears as it is just the A.I. creating an anomaly without realizing it's nature to those who would encounter it, in a similar manner to other anomalies in the original Super Mario 64 such as the Wario Apparition.