Final answer:
The Arctic mythology deity responsible for sea mammals is likely Sedna from Inuit mythology, not a figure from the provided Greek mythological excerpts.
Step-by-step explanation:
The goddess responsible for the origin of sea mammals in Arctic mythology is not specifically referenced in the provided passages, which instead focus on ancient Greek mythology. Characters such as Poseidon, the god of the sea, and his children the Nereids, as well as other deities related to the sea like Thalassa and Gaia, are depicted. However, Arctic mythology, which typically pertains to the indigenous peoples of the Arctic regions, such as the Inuit or other Native American tribes, might have different mythological figures. Sedna, also known as the Mother of the Sea or Mistress of the Sea, is a prominent figure in Inuit mythology and is often associated with the sea and its creatures.