[Horos]

All the constants are considered part of the pantheon (strictly speaking Amier was part of their pantheon and split etc etc)

Amier/Ymir can be regarded a member of the Pantheon because all the others are derived from him. 

However, there seems to be no lore connection between the rest of the constants and the pantheon, (except for Mortyl and Caius having children and Pylae + Unknown.) From a lore standpoint, this connection doesn't make much sense.