One kind of Agni(fire). The dictionary meaning of Āyu (Ayu) is ‘life-force’. The Agni that abides in the body of living animals as a life-force, is referred to as ‘Āyu-Agni’ (Ayu-Agni)
In Vāyupurāṇa (Vayupurana) it is said that Āyu-Agni is the son of Śuci-Agni (Suchi-Agni) and father of Mahimān (Mahiman). According to Brahmāṇḍapurāṇa (Brahmandapurana), Āyu-Agni is the father of Mahiṣa (Mahisha).