In the script for “The Day the Earth Stood Stupid,” written by Jeff Westbrook (with a story by Westbrook and David X. Cohen), the Hypnotoad was merely described as a huge toad with hypnotic eyes. Ervin looked to the Argentine horned frog as a model, a round, squat amphibian also known as a Pacman frog for its resemblance to the Namco arcade star. The Argentine horned frog also has an enormous mouth. The Hypnotoad sits a little more upright. With a bit of embarrassment, Ervin noted that the Hypnotoad was not based on a toad at all. 

For the hypnotic eyes, Ervin recalled a very specific moment from “The Simpsons” as inspiration, seen in the picture above. The eerie eyes of the Hypnotoad were meant to imitate Bart Simpson’s eyes when he ate too much sugar. In Ervin’s words: 

“As for his eyes, I looked at those oddly-shaped eyes that a frog has and based it on that. Also, I’d previously worked on ‘The Simpsons’ episode ‘Boy-Scoutz ‘n the Hood,’ where Bart and Milhouse get all amped up on Squishees and have these crazy eyes. It was a bit of a nod to that.”

“Boy-Scoutz ‘n the Hood” first aired on November 18, 1993 during the show’s eighth season.