According to Bhāgavata Purāṇa (Bhagvata Purana), the sons born out of the womb of Sarupā (Sarupa) impregnated by the semen of Bhūta (Bhuta)  are known as Rudra. Ugra is one of these Rudra(s).

Bhāgavatpurāṇa Vols. 1-12(6.6.17)