Flood San 'Shyuum Form

Halo 2
The prophet form may not be an actual form of flood, as the bio-mass of Prophets is quite small. Instead, prophets may become a Carrier Form. However, the Prophet of Regret is seen as a semi-assimilated part of Gravemind during Master Chief and the Arbiter's encounter with the Gravemind. It is similar to the prophet, but a tentacle reaches through the prophet's main mass, and the skin does seem somewhat decayed.

Halo Graphic Novel
Another Prophet form can be seen in the Halo Graphic Novel. After Half-Jaw departs for the engineering bay of the Infinite Succor, the commander sees the infected Legate, taunting him. The Legate has become part of the massive infection that took over the vessel. The Prophet could have become part of the Brain Form as the Flood were gathering many bodies and body parts for the large biomass.

Communication
It is strange however that the Legate communicated directly with the Commander instead of the biomass itself. In Halo 2, the Gravemind speaks for himself and uses 2401 Penitent Tangent and the Prophet of Regret as tools of knowledge, not to speak on behalf of the Flood. Also in Halo 1, Captain Keyes and the other infected units on the bridge of the Truth and Reconcilation did not speak or interact, yet the infected Legate does.

It should be noted that the existence of this Flood is still met with skepticism