I'm told the original creatures game is on GOG, but when I search all I can seem to find are the later ones in the series like Exodus. Is the game actually unavailable, or has it done the thing where the first game has a subtitle? If the latter, which game is the original?

Great question! The original Creatures game, often referred to Creatures 1, is included in Creatures: The Albian Years. This includes both Creatures 1 and Creatures 2. Just in case you're interested, Creatures Exodus includes Creatures 3 and Docking Station. Hope this helps you out! Enjoy!