Frostgoblin's page

Missed that part since it wasn't in the Eidolon section of AONPRD.

My statement about Beast and Plant Eidolon still stands, which is a weird case. Also everything else still stands.

Yeah, I don't know how this interacts with the beast and plant Eidolon, what happens when you encounter your own unmanifested Eidolon just "living" their life (using the broadest sense of living since the majority of the Eidolons available aren't really alive)? But about the dragon Eidolon that has an explanation as Creative Burst very helpfully pointed out:

"Because dragons have a strong connection to magic,

their minds can often leave an echo floating in the
Astral Plane. Such an entity is extremely powerful but
unable to interact with the outside world on its own.
Dragon eidolons manifest in the powerful, scaled forms
they had in life;"

Each Eidolon types gives a description of what it is or how it came to be.

So it is less a dragon from the astral plane and more of a dragon's psychic impression left on the astral upon death.

Reading the entry on Eidolon on the Archives of nethys reveal that each Eidolon has a home plane where they return to when unmanifested (thus making Eidolon eligible for banishment on the material plane), so technically the summoner DOES summon the Eidolon even if it is not tagged as a Summon.