All Vedic Gods, Vishnu, Indra, Agni, Rudra, Varuna, etc. are mentioned in the RigVeda as Asuras.
तवमग्ने रुद्रो असुरो महो दिवस्त्वं शर्धो मारुतं पर्क्ष ईशिषे l
तवं वातैररुणैर्यासि शंगयस्त्वं पूषा विधतः पासि नु तमना ll
Tvamaghne rudro asuro maho divastvaṃ śardho mārutaṃ pṛkṣa īśiṣe l
tvaṃ vātairaruṇairyāsi śaṃghayastvaṃ pūṣā vidhataḥ pāsi nu tmanā ll
Thou, O Agni, art Rudra, the Asura of the high Heaven; thou, being the host of the Maruts, rulest over nourishment. Thou goest along with the flame-coloured Winds, bringing happiness to our home. Thou, being Pûshan, protectest thy worshippers by thy own might.
The Rig Veda in Sanskrit: Rig Veda Book 2: Hymn 1