The Classic Skins Gamepass is a Gamepass available for purchase from the Dinosaur Simulator gamepass store (the “Store” tab next to the “About” tab; this applies to all Roblox games with gamepasses/gear). It was the most expensive gamepass before the Kaiju Spinosaurus price buff, available for purchase at the price of 
450. It’s even more expensive than the limited WDC Donations Gamepass which cost 400.
Function
This gamepass unlocks the former, classic skins, for dinosaurs such as Tyrannosaurus, Triceratops, and Brachiosaurus, which existed in the game before the major graphics update and switch to unioned models. By purchasing the model, 34+ classic skins are unlocked and available for use at the time of release.
Implementation
Because of the fact that classic skins were free and of a lower quality and playability than the base models of certain dinosaurs, ChickenEngineer didn’t want new players selecting old, outdated models when playing as dinosaurs that had more advanced and fluid movements on the base models. Rather than remove the skins entirely, he opted to make the skins purchasable, putting them in a gamepass and offering the ability to play the classic skins to older players.