You haven't mentioned but I assume that you play on PC and not on a console.
It's not unusual for developers to leave unfinished material, characters, items, audio, maps and so on, within the code of any game and PC gamers have gotten quite good at datamining to find this material and access it.
While I cannot speak with any authority but the "new" goat may have been an abandoned prototype for the Hover Goat 3000 which was abandoned for the current design but left locked away somewhere within the code of the game.
Likewise the Delivery Chomper currently exists but is locked to a certain game mode, the Delivery Missions, but it would be relatively simple for someone to tamper with the game code on their PC to bring him into regular play. Last year we had a similar situation where a handful of PC gamers were unable to unlock the RoboCat and Junkasaurus characters fro Infinity Time and bring them into multiplayer games. Likewise many PC gamers were able to acquire the Torchwood and actual Hover Goat 3000 weeks before anyone would be able to have legitimately unlocked them for play and have also been able to unlock the new "alternate" character abilities before they were released.