In "Land Before Swine." After Stan and Waddles fall off the track, the egg in the nest hatches and out came a little pterodactyl hatchling. Everyone is moved by it's cuteness at first, however, just as Old Man McGucket welcomes it to the world. The hatchling quickly swallows him whole (but he survives).
Dipper, Soos, Mabel sit completely still while the hatchling feeds on the egg shells, Soos then thinks they should stand in a straight line as he explains that it's eye are both crossed so it wouldn't be able to see them from the front. They do so and successfully escape the hatchling before it turns away.
Later McGucket makes it back up, having eaten his way out of the hatchling itself
- 118. Land Before Swine