Futurama's Hypnotoad design was a subtle nod to The Simpsons

In the screenplay for "The Day the Earth Stood Stupid", written by Jeff Westbrook (with a story by Westbrook and David X. Cohen), the Hypnotoad was described simply as a huge toad with hypnotic eyes. Ervin turned to the Argentinian horned frog as a model, a round, stocky amphibian also known as the Pacman frog for its resemblance to arcade star Namco. The Argentine Horned Frog also has a huge mouth. The Hypnotoad is a little straighter. With some embarrassment, Ervin noted that the Hypnotoad wasn't based on a Toad at all.

For mesmerizing eyes, Ervin recalled a very specific moment from “The Simpsons” as inspiration, seen in the image above. The Hypnotoad's weird eyes were meant to mimic Bart Simpson's eyes when he ate too much sugar. In the words of Ervin:

"As for his eyes, I looked at those oddly shaped eyes that a frog has and based on that. Also, I had previously worked on the episode 'Boy-Scoutz' n the Hood" from "The Simpsons" where Bart and Milhouse get excited over Squishees and have those 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.