The Poseidon Subclass Light Battlestar is an modified version of the Freyja Subclass Light Battlestar which in turn is a subclass of the Valkyrie Class Light Battlestar. While the Poseidon looks similar to the Freyja. it has been modified to be able to launch missile from it's top side. This missile can either hit space based ships, space stations or planetary targets. Origionally, 12 ships of this class were requisitioned but only seven ships were built before the remaining five were canceled.
The Poseidon was going to be the ninth Valkyrie Class Light Battlestar and the Xanthus was to be the eleventh, however while they were under construction, they were selected to be modified to become a new subclass. Delays in funding led to the Freyja becoming that subclass. The Poseidon and Xanthus were selected to become the second and third Freyja Subclass Light Battlestar. During their construction Admiral John Kronus proposed the ideas of a Battlestar with the ability to launch missiles at both planetary targets and space based targets. The Poseidon and Xanthus were selected to be modified into the new class. After the Poseidon and Xanthus were finish and the Bellerophon, Odin, Celestial, Ramses, and Icaris had been started, the public outcry became so great the Admiralty canceled the five remaining unnamed Poseidon Subclass Light Battlestars. The six Poseidon Subclass could only carry three Viper Squadrons (120 Vipers) and 30 Raptors.
Because of the outcry only seven ships were built and all had been decomissioned by the time of Operation Homecoming/Downfall however some rumors seem to indicate that some have found their way into the hands of Admiral Arthur Wallace.
Poseidon: Retired before the new Poseidon, a Poseidon Class Heavy Battlestar entered service.
Bellerophon: Bellerophon was only in service for two years until it was decommissioned. The name was used on the Achilles Class (Hero Class) Battlestar.
Celestial: Decommissioned after eight year of service. An explorsion destroyed the port side flight pods and it was decided to decommission the Celestial instead of reparing it. The name was used for the new Durga Class Battlestar.
Icaris: Icaris's top side launch system was removed and in it place a Raptor landing pad was added. The first battlestar to carry a Raptor landing pad.
Images is taking form Starship Schematics Database.