Syncretism

Syncretism is the merging or assimilation of originally discrete traditions, especially in mythology and religion; this melding of cultures effects the mythological entities (especially the deities) represented by those cultures. Syncretism can result in gods from one culture being adopted into a foreign pantheon, intermarriage between gods from different pantheons, and the creation of new syncretic deities.

Causes
This can happen from two religious groups or peoples simply existing in proximity to one another and interacting or intermarrying over an extended period of time. It can also also happen when a culture is conquered and the the conquerors bring their culture with them but do not succeed at eradicating the native religion (or don't attempt to).

Greek-Egyptian Syncretism
During the Ptolemaic period for instance Isis was married to the Greco-Egyptian god Serapis as part of an attempt to reconcile and unite the two pantheons. This union produced the god of silence Harpocrates.

Greek-Celtic Syncretism
Several Celtic goddesses such as Sirona, and Rosmerta, also married the Roman gods after the conquest of Gaul and Britain.