One of the most iconic monsters in Dungeons & Dragons is the gelatinous cube. Imagined by creator E. Gary Gygax himself in the very first edition of Dungeons & Dragons in 1974, the gelatinous cube is a staple foe that players encounter early on in their adventuring. (It has an entry-level challenge rating of "2" in the current version of D&D. That's low.) The cube moves slowly but can be very dangerous once it gets close, as it absorbs victims and slowly digests them while they float helplessly inside.
The Burger Shire, a fast-food restaurant, has a sign reading, "Now serving 2nd breakfast." In the Lord of the Rings books and movies, second breakfast is a meal enjoyed by the Hobbits, who all live in The Shire.
Pixar's iconic Pizza Planet truck appears in the troll bridge toll scene.
The popular "A113" reference is present in this film as it is in all Pixar films. The Police call that comes through near the end says, "A 113 in progress." A113 was the classroom used by John Lasseter, Tim Burton, and Brad Bird at the California Institute of Arts and appears in all Pixar films. This is the first time that it only appears verbally.
In an email to Inverse, Wizards of the Coast confirmed it collaborated with Disney and Pixar over the use of D&D characters and story elements in Onward. "The D&D team was super excited to meet with the writers and producers of Pixar's Onward," Wizards of the Coast told Inverse. "There was a lot of back-and-forth in the room discussing how best to portray D&D monsters like the beholder and the gelatinous cube. We love that Onward is bringing fantasy to a whole new audience, and it's a testament to how D&D storytelling is part of the mainstream culture now."