Asura, Rakshasas have been mentioned even in Rig Veda (I.35.10) also. However, the word Asura was used, mostly as an epithet, for Vedic deity, such as Savitr. Rakshasa was used to indicate an epithet.
May he, gold-handed Asura, kind Leader, come hither to us with his
help and favour. Driving off Rākṣasas and Yātudhānas, the God is
present, praised in hymns at evening.
1) In Ramayana the word Rakshasa was used to indicate man-eater (पुरुषादम्) also. Viradha, Kabandha were such Rakshasas, who confront Sri Rama, in Aranya Kanda.
2) 32 Crores of Rakshasas were living in Lanka under the leadership of Ravana.
दश राक्षस कोट्यः च द्वाविंशतिः अथ अपराः |
वर्जयित्वा जरा वृद्धान् बालान् च रजनीचरान् || ३-५५-१४
तेषाम् प्रभुः अहम् सीते सर्वेषाम् भीम कर्मणाम् |
"Oh, Seetha, ten crores of first-rated demons are there, furthermore,
twenty-two crores of demons of ghastly exploits are there, thus all
put together, three hundred twenty millions of demons are there in
Lanka, omitting the weakish, oldish, youngish demons. For all of them
I am the lord." Thus Ravana started his self-eulogy.
So Rakshasas used to live either independently or in groups during Ramayana period.
In Ramayana, Asuras was used to indicate another group of demons, contrary to Vedic literature.
Minaka, who rose from Ocean to give hospitality to Sri Hanuman, was stated to have been obstructing the entrance to Patala, which was the residing place for Asuras.
त्वमिहासुरसंघानां पाताळतलवासिनां || ५-१-९२ देवराज्ञा गिरिश्रेष्ठ परिघः
"O best among mountains Mainaka! You have been positioned here by
Devendra as an obstacle to the groups of asuras living in paataala."