As of now, we know that Blackbeard has two Devil Fruits within him, the Yami Yami no Mi and the Gura Gura no Mi. There are certain beliefs that prior to obtaining these two, he has a Zoan-type fruit. A possible name for the could be: Inu Inu no Mi, Model: Cerberus.
First of all, a quick review on what exactly a Zoan is. A Zoan is one of the three types of Devil Fruits along with Logia and Paramecia. It allows the user to transform into a different species or a hybrid-form of that said species. There are many types of Zoans:
Inanimate Object Zoan
Due to Dr. Vegapunk's research, certain inanimate objects have been shown to eat Devil Fruits. Such examples are Lassoo and Funkfreed which are Mr. 4's bazooka and Spandam's sword respectively.
|Inu Inu no Mi, Model: Dachshund|
|Zou Zou no Mi|
The Awakened Zoan are users who have attained the ability to be stronger, faster and tougher than what the Zoan Devil Fruits' abilities bestow normally. This also includes faster recovery time, which functions involuntarily even when the user is unconscious. Such users are the four Demon Guards of Sadi-chan in Impel Down.
|Minozebra, Minokoala, and Minorhinoceros with Sadi-chan|
The Mythical Zoan are a very rare but very powerful type of Devil Fruit. It allows the user to transform into mythical creatures like those one would see in legends. Only two users have been shown as of now, Marco of the Whitebeard Pirates, and Sengoku of the Marines.
|Marco, 1st Division Commander of the Whitebeard Pirates|
|Marco as a Phoenix|
|Marco in his phoenix-human hybrid form|
|(Former) Supreme Commander of the Marines, Sengoku|
|Hito Hito no Mi, Model: Daibutsu|
The most appropriate type of Zoan for the Cerberus Theory is the Mythical Zoan. For one, it is considered to be a rare type of creature and its power is immense.
There are certain instances that back-up this claim. The Blackbeard Pirates' Jolly Roger, Shanks' wound, and the mere Existence of a Cerberus in the One Piece World.
The Jolly Roger, or the Pirate Flag, help determine who belong to whose crew. The designs usually reflect the interest of the captain. At other times it may be physical resemblance or the symbols behind them. In some sense, there is a reason or story for every Jolly Roger design.
For some reason, the design of the flag of the Blackbeard Pirates show three (3) skulls side by side. There hasn't been any explicit reasons for this but it might be revealed.
Due to this, some think that the flag resembles the skull structure of a Cerberus.
One other instance that aids the Cerberus claim, it is the wound on the left eye of Shanks. It consists of three diagonal scratch marks.
The mere stating and showing of its name and being respectively signals belief in its existence.
Well somehow, this quote seems to be true. In Chapter 444 there was shown a Cerberus in Thriller Bark.
|One of its heads is a wolf|
There was another instance wherein a Cerberus was mentioned. This was during the CP9 Arc in Enies Lobby courthouse. In Chapter 387, during the Battle between the Franky Family and Baskerville, there was a comment on how Baskerville resembles a Cerberus.
|Zambal calling Baskerville a Cerberus|