The creature known only as the "big yellow thing" is the first monster to emerge from The Trap Door in the show's first episode "Breakfast Time". This creature is mostly a large head, pushed along the ground by two bent legs at the back of its squat body.
In the episode "Breakfast Time", the yellow thing emerges from the trap door as Berk opens it to let out some worms (for future cooking). The creature spots a heap of pastries that Berk had just prepared as breakfast for The Thing Upstairs and is quick to devour the lot of them. Later, when Him Upstairs demands what's keeping his breakfast, Berk drags the sleeping yellow monster to the service lift and serves it as breakfast for his master. Him Upstairs found it to be most delicious.