The Pachycephalosaurus (thick-headed lizard), or simply referred as Pachy, a medium-sized herbivore is light grayish brown in base color with small yellow spots scattered near its belly. Its feet have three grey clawed toes. Its neck, spiky forehead, and middle tail spine are light brown colored. The beak and eye spots are yellow in color, same for the rest of the tail, with most of it being yellow-colored as well and a tail tip the same color as the base color.
- While the young Pachycephalosaurus in-game is just a smaller version of the adult, it has been theorized by Jack Horner and other paleontologists that Stygimoloch and Dracorex are younger Pachycephalosaurus.
- When the creature first was added, ChickenEngineer made a tiny mistake in the code, and Pachycephalosaurus' elder damage was 900, higher than any other dinosaur. Servers were initially ravaged by people killing everything with them.
- It was also known that originally, its Growth Rate Multiplier was 1.0 before it was decreased to 0.4.
- Pachycephalosaurus has recently been theorized to have been an omnivore throughout some of it's lifetime due to a partial juvenile skull with theropod esque teeth being discovered.