Giant bullfrogs are a bonus non-adoptable animal that can be unlocked by obtaining a low score in the Extinct Research Labcloning minigame. They act as ambient animals when released from their crates but, like all ambients, they eventually disappear. The Giant bullfrog looks just like the regular ambient bullfrog ambient from Endangered Species, but is much bigger. They are also the most common ambient animal, and they are the only ambients found in the Extinct Animals Demo.
Giant amphibians did indeed exist in prehistoric times, but most were not as large as the Giant Bullfrog. It is possibly a Beelzebufo.
If a child guest sees the giant bullfrog, they will sometimes laugh at it.