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. With the Poseidon looks similar to the Freyja is has been modified to be able to launch missile form it top sides. This missile can either hit space base ships or space station and planetary targets. Only seven ships was built before this remain five was cancel.
The Poseidon was going to be the ninth Valkyrie Class Light Battlestar along with the Xanthus what was going to be the eleventh, however while they was under construction, they was selected to be modifited into a new Subclass. However delay in funding led to the Freyja become that subclass. So the Poseidon and Xanthus was selection to become the second and third Freyja Subclass Light Battlestar. However during their construction Admiral John Kronus proposed the ideas of a Battlestar with the ability to launch missile at both land base targets and space base targets. So the Poseidon and Xanthus was selected to be modified into the new class. After the Poseidon and Xanthus was finish and the Bellerophon, Odin, Celestial, Ramses, and Icaris had been started the publics outcry became so great the Admiralty cancel the five remain unnamed Poseidon Subclass Light Battlestar. The six Poseidon Subclass could only carry three Viper Squadrons (120 Vipers) and 30 Raptors.
Because of the outcry only seven ships was built and all have been decomissioning by the time of Operation Homecoming/Downfall however some rumors seem to said that some have found there way into the hand of Admiral Arthur Wallace.
Poseidon: Retire before the new Poseidon, a Poseidon Class Heavy Battlestar enter service.
Bellerophon: Bellerophon only service for two years till it decommissioning. Name was ued on the Achilles Class (Hero Class) Battlestar.
Celestial: Decommissioning after eight year of serviced. An explorsion destroyed the port side flight pods and it was decide to decommissioning 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 Raptor landing pad was added. The first battlestar to carry Raptor landing pad.
Images is taking form Starship Schematics Database.