Try /pet remove on another game mode. /pet hide just hides the pet on whatever game mode you're on which is why the lobby didn't work.
Slime stalker.. creepy. It's so frustrating, I have to go on Skywars to remove it and I hate it so much.. All gamemodes should allow pet commands
Many of the pet commands were causing significant lag as reported by the server timings. By removing access to some commands that call the database such as removing pets from individual servers, the lag went down significantly. You'll need to do /pet remove in the lobby for it to be removed on all servers.