Tetrapodophis only makes a brief appearance when Arlo and Spot encounter it on their journey. It first appears when Arlo starts eating berries and Tetrapodophis falls on him and Arlo shakes it off. Confronted by the Tetrapodophis after falling off the cliff, Spot was able to wrestle it to save Arlo from the predator, thus retreating. It is never seen again for the rest of the film after this.
Several Tetrapodophis appear as non-playable characters throughout the game. They are seen as obstacles that the player has to avoid. Depending whether the player is controlling Arlo or Spot individually or controlling Arlo and Spot together and he/she touches one, touching a Tetrapodophis either loses a life or causes an instant game over.
Tetrapodophis was discovered in early 2015, a few months before The Good Dinosaur's release.
He just mixed his alignment of bad and neutral while just a predator and was not fully neutral.