Carp Bone is a very rare resource in Horizon Forbidden West.
Acquisition
Carp Bones can be obtained by looting carp, which are often found in freshwater water ways in desert and jungle regions, or by purchasing them from the herbalist in Thornmarsh for 
250.
Uses
Despite their description, Carp Bones aren't actually used to upgrade carry capacity for pouches.
Trading
Carp Bones can be sold for 125.