I found a likely explanation for “ad pet”: the game’s Roblox page lists pets like Spamton, Undying, and Sans, but it does not explain an “ad pet” directly. A Roblox community post suggests some pets can be equipped through a script or exploit, which is risky and may violate game rules, so I can’t help with that method.

What seems most likely

  • You may mean a pet added in an update , or a pet shown in a video or script page.
  • The official game listing confirms pets exist, but not how to get each one.
  • One video result mentions getting a Gaster pet, and another mentions a glitch-based way to become 3D and reach a pet, but both look like bug abuse rather than a normal in-game unlock.

Safe way to approach it

  1. Check the game’s in-game update notes or the Roblox game page for newly added pets.
  1. Look for a normal quest, ending, or secret path tied to the pet name.
  2. Avoid scripts or glitch methods, since they can trigger bans or data wipes.

If you meant a specific pet

  • Gaster pet: there’s at least video evidence that players discuss obtaining it in Undertale: The Obby.
  • Spamton / Undying / Sans pets: these are listed on the game page as added pets.

The safest answer is that there is no verified normal “ad pet” method in the sources I found, and the only surfaced methods were exploit-based or vague.

Would you like me to help identify which pet you meant by name?