Hi guys I am rereading the entire series from PJO to ToA and I noticed something connecting them all, besides the fact that they're in the same universe and continuations and whatever. (MC and KC are not included). Rick has successfully introduced one type of monster every book and kind of focused on it. Like take this, in The Lightning thief, it was the Furies, in Sea of Monsters it was Cyclopes, the Ophiotaurus / manticore in TTC, empousai in tbotl and laistrygonian giants in last Olympian, even in lost hero it was venti, the grain spirits in SoN, Skolopendra/ichthyocentaur in MoA, pretty much every monster in HoH but maybe Drakons in particular, and Lares in TBoO. And, in ToA, in tHO, it was myrmekes, blemmyae in Dark Prophecy, and strixes in Burning Maze. These are not just any monsters that appeared in the books, they appeared a lot more times than any other monsters in their given books, or in more numbers. Except the last Olympian and tBoO, where there were armies of monsters, in which case these were simply mentioned for the first time, or actually fought one on one with a main character.

I'm sorry this is so long, and if you find any other monsters, feel free to name them here.