According to Vāyupurāṇa (Vayupurana), Āpa(Apa) is a rākṣasa (rakshasa; a type of monster). He lives in the chariot of Surya (the Sun-god) for the two months, Āśvina(Aswina) and Kārtika (Kartika).

Brahmāṇḍapurāṇa (Maharṣi)(1.23.15)
Vāyupurāṇa(52.15)