Nag, and his wife Nagaina, are snakes. Specifically they are cobra snakes.

What is nag snake in English?

/nāga/ mn. cobra countable noun. A cobra is a kind of poisonous snake. /naga, nAga, naaga, nāg, nag, nAg, naag/

Who is naga Devi?

of nāga; IAST: nāgī; Devanāgarī: नागी) are divine, semi-divine deities, or a semi-divine race of half-human half-serpent beings that reside in the netherworld (Patala) and can occasionally take human form. ... A female naga is a "Nagi", "Nagin", or "Nagini". Nagaraja is seen as the king of nāgas and nāginis.