Between The Percy Jackson and the Olympian series, The Heroes of Olympus series and the Trials of Apollo series, multiple armies have shown up.
The Titan Army consisted of The Elder Titans (though Koios is never mentioned by name nor are battles he fought in and Iapetus is barely part of it before losing his memory) as well as Atlas and Prometheus (Prometheus acting only in a diplomatic fasion). The Titan Army also consisted of several minor gods (Hecate, Melione, Nemesis, Janus Morpheus Aigaios. The army also consisted of human mercenaries, demigods like Luke who felt abandoned by their parents or that the ones like Ethan Nakamura who joined because they Olympians did not show their parents the respect they deserved and many monsters some of whom had grudges against the Olympians such as Kampe others such as the Keres who were promised more liberties and other monsters like hellhounds who fought because it was in their nature.
The Giant Army was led by the Gigantes (100 monsters who with the exception of Alkyones could only be killed by the combined efforts of a god and a hero). The Giant Army consisted of many members of the defeated Titan Army such as the (Hyperborean Cyclops and Empousa) as well as new gods such as (Boreas, Melione & Nyx) spirits (Venti and Karpori) immortals (The Boreads Zethes and Calais), Souls that had been raised from the dead such as Medea & Phineas) and others who were shades (they were unlike Medea and Phineus) but had been resurrected by Gaia.
The Olympian Army consists of many branches. One part is the Roman demigods from Camp Jupiter which included Giant Eagles and an elephant Hannibal. the second is the Greek demigods from Camp-Half Blood. Also from Camp Half-Blood there were Pegasi, Satyrs, Nature Spirits and while they are not at Camp-Half Blood, The Hunters of Artemis added to the forces. Chiron and the Party Ponies added to their numbers. Percy was able to buy help from the god of The East River and The Hudson River with a sand dollar. The Olympian forces under the sea included The Olympian and Lord of the Sea Poseidon along the goddess Amphitrite, and the gods Triton, Delphin and Palaemon. The forces of the sea also consist of the Cyclops Army, Briarias and The Merpeople Army and other creatures of the sea i.e. Dolphins, Sharks ect... The Olympian forces also included Hades (who having finally been convinced by his son Nico) came with Demeter and Persephone and an army of undead warriors (from all time periods). Finally there were the Olympians themselves. After the Second Titan War former deities who were part of the Titan army joined the Olympians such as Hecate and Nemesis even Iapetus helped Percy and Annabeth through Tartarus (though for a good portion of the time he did not remember having done bad things).