In "Land Before Swine." After Stan and Waddles fall off the track, the egg in the nest hatches and out came a little pterodactyl chick. Everyone is moved by it's cuteness at first, however, just as Old Man McGucket welcomes it to the world. The chick quickly swallows him whole (but he survives).
Dipper, Soos, Mabel sit completely still while the chick 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 check before it turns away.
Later McGucket makes it back up, having of eating his way out of the chick itself.
- 118. Land Before Swine