Am I the only one who noticed how some things were just added and never mentioned or vice versa? Like, look at the biggest example:
Storm spirits were never seen until TLH and then were mentioned in almost every book and even major characters.
Other examples:
Harpies are mentioned since they "clean" CHB, but then Ella, a harpy, becomes a major character in HOO and TTT.
The wind gods were only referenced in TLO, but then they are seen in almost every book of HOO and even harnessed for Zeus' chariot.
The Minotaur was major in PJO, TLT, and TLO, but then never seen again, only referenced in 3/5 of HOO books, and never seen nor mentioned again.
Telekhines are shown fairly majorly in BOTL and TLO but almost never seen again in HOO, just minimally shown in MOA.
Basilisks were never mentioned nor shown in PJO but in SON and BOO of HOO, they are shown and fought by Frank and Jason.
The Chimera was shown in the very first book TLT, but then never seen nor mentioned, although since she was with Echidna and she was never mentioned, it makes minimal sense.
This one doesn't really bother me despite the implementation of them, but Katobleps, cow-like monsters are never seen nor shown until HOH, and as soon as they are shown, Frank jumps in and kills all of them for a snake.
Yales, and this one is bothering me, are never mentioned at all but are then suddenly used as an obstacle for Meg and Apollo in TDP.
There are multiple creatures and beings and even places, such as Aeithales and Aeolus, the god-emperors (never mentioned and are suddenly behind the past two series and have been working to destroy the McLean's! Still great villains), Python, and found Oracles, but I digress.