The Anuboo, the shapeshifting jackal ghost "taht haz lazr vishun"* in Tee Hee Valley. In order to understand this theory, let's decipher its name.
Anuboo > Anubis Boo > Boo > Boo = ghost sound > Ghost > People/Animals that died
Anubis <----|
Anubis is an Egyptian God with a jackal head. Jackals are canines. As are dogs. The point?
Anuboos are dead jackals or pampered dogs. If they died of old age, nothing but an animal with a purpose. But if Bowser/Cackletta were involved ... ... then they should never come back, if you understand.
*Intended lolspeak.