Tristan

Tristan the Whalemingo is an alien creature that appeared in the episode When Worlds Collide.

Tristan and Buford met when the latter was walking through the forest at night with his lucky tambourine. He quickly gained the whalemingo's trust with a pack of shrimp jerky that he had in his pocket, and the two bonded. Eventually, though, Tristan had to return to his home planet.

In the episode, the other children, Baljeet especially, doubt Buford's story of his alien encounter until the Whalemingo planet, drawn in by Doofenshmirtz's invention, begins hurtling towards Earth. Buford immediately recognizes Tristan out of the thousands of other Whalemingos and at the end of the episode, gifts him his lucky tambourine as a token of their friendship.

Whalemingos
A Whalemingo, like its name implies, is a combination of a whale and a flamingo. Whalemingos have a big whale-shaped body, the head, neck and legs of a flamingo and are covered in pink feathers. Normal flamingos are pink because of the shrimp they eat, so it's likely that Whalemingos eat the same thing.

All Whalemingos, Tristan included, live on a massive pink planet, were they swim and eat in massive jungles.