Tazzie is a Thylacine (or Tasmanian Tiger) who is one of the secret agents of Phineas and Ferb (fanon).
He is one of the few animal agents who works outside O.W.C.A.. In fact, he is a debated mystery in the Agency. He has rarely been seen by anyone as he and other members of his species went into hiding when thylacines were being hunted to near extinction. Sightings of Tazzie occur when he occasionally comes out into the open, usually when he goes missions. It has been long questioned as to whether he is good or evil.
He first appears in the special fanon episode Phineas and Ferb: Going Down Under. He is first mentioned by Major Monogram, Carl, and the other agents; and Perry decides to try tracking him down when he goes on a mission in Australia (where the Flynn-Fletcher family coincidentally goes on vacation).
Perry the Platypus
He is discovered by Perry in Phineas and Ferb: Going Down Under. At first, they endure in a fight and seem to hold grudges against each other. Later, he teams up with Tazzie to defeat Doofenshmirtz. At the end, Tazzie and Perry get along, and Perry salutes to him as he goes back into the bush.
The only one other than Perry to know of his existence, Tazzie seems to like, or at least, tolerate him because they are both supposedly "extinct" species.