All Seals Guardians's name end with the suffix -eos, which is derived from the latin word for God. This was done on purpose to divinify those Pokémon.
Nepheleos would therefore be considered God od the Sky (despite being associated with the ocean rather than the sky in-game).
Nereideos would therefore be considered God of the Ocean (despite being associated with rivers and lakes rather than the ocean in-game).
Lampadeos would therefore be considered God of the Underworld (despite being associated with the desert rather than death in-game).
All three Seals Guardians were originally associated with a different area, this is because the inspiration for the region shifted during development but the Guardians's name and appearances were kept. Only their location of choice was modified.
Nepheleos was once supposed to act as duo master for the other two Guardians, but that concept was scrapped.