Each of the horses had been given a preparation mixed by Franck's Pharmacy in Florida.
Necropsies performed on the horses revealed significantly increased selenium levels in samples tested.
"Signs exhibited by the horses and their rapid deaths were consistent with toxic doses of selenium," state veterinarian Dr Thomas Holt said following the examinations.
The horses, from the Venezuelan-owned Lechuza Caracas team, began collapsing on April 19 as they were unloaded from trailers at the International Polo Club Palm Beach before a championship match.
Franck's Pharmacy chief operations officer Jennifer Beckett said last April that the strength of an ingredient in the preparation was incorrect, but would not name it.
The court action, in Palm Beach County Court, has been filed by Quantum Management Company and three player/owners, Juan Martin Nero, Guillermo Caset, and Nicolas Espain.
They alleged the pharmacy was responsible for improperly mixing the compound, leading to the deaths.
The lawsuit is seeking more than $US4 million. It alleges the pharmacy used too much selenium.
It has yet to be revealed how the litigants arrived at the damages claim of $US4 million, but top class pony ponies can be worth upwards of $US100,000.
The deaths of the ponies resulted in the US Polo Association conducting a wide-ranging review of pony health, safety and welfare, resulting in new drug regulations and a drug-testing programme effective from this season.
The horses that died were Turca, Shakira, Platera, Cocata, Teletubi, Lunita, Xuxa, Federica, Abrigadita, Julieta, Nina, Mojarritta, Minibank, Princesa, Vasuda, Gausuncho, Pelusa, Holandesa, CanCan, Monzon and Oracion.
Authorities are continuing their investigations into the deaths.