Terrybubble's form is that of a duckbill-type dinosaur, with a flat head, a long snake-like neck, short front legs and long back legs, and a powerful tail. Speedy calls it "a bit kangerooish" in appearance. When it is re-awakened to life, it is a moving skeleton, with "bright and intelligent balls of phosphorous" for eyes.
In personality, Terrybubble is friendly, talkative, and inquisitive, like a bright child. The creature does its best to imitate a pet dog for Speedy; in high spirits, it performs wild impetuous dances it calls "dythrambs."
In its original incarnation, the creature lived "thousands" of years ago in the "mezozoic" era, in the Valley of Virtula (located in modern-day Wyoming). At the young age of 400 years, it was killed when a "mogerith" bit its neck. Eons later, its bones were discovered by a prominent paleontologist, and restored to their proper arrangement by Speedy and his Uncle Billy. Then a massive geyser eruption, a "smoking, roaring phosphorescent torrent," blasted boy and skeleton upward toward Umbrella Island. Lingering magic in Speedy's wishing powers, combined with the mysterious force of the geyser, united and animated the bones into a living, talking being. (Speedy in Oz)
As they were soaring through the atmosphere, Speedy remarked on his ordeal, "This is terrible." Yet because of his chattering teeth, his verbatim statement was "This is terrybubble." His listening companion, newly awakened to life, took this as its name.