Poseidon is a deusphage and large sized tank-like Aragami resembling a Tezcatlipoca first introduced in God Eater. Just like all deusphages, the Poseidon was also introduced in God Eater via DLC content. It returns in God Eater Burst only as a regular Aragami.
The following information is taken from the offical in-game descriptions of the Aragami found in the NORN Terminal's database.
God Eater Burst
A Type 2 Deusphage derived from the Quadriga. It boasts ruthless flames that will scorch the earth the moment it unleashes its powers.
Its defense is literally iron-tight, but the missile pod to the sides and the front armor is relatively weak. After destroying the cell binds, use sundering and crushing blade attacks.
Poseidon is the god of the sea in Greek Mythology. However, the Aragami of the same name uses Blaze, which could be based on the term "sea of fire".