Are there any avatars of Devi which are not mentioned in Vedas but there is a mention of them in puranas. Also are there any avatars, which are not a part of puranas and itihasas but in shrutis(Vedas, upanishads) ?
Rig Veda mentions many female deities. Ushas is the most celebrated goddess along with Aditi, Prithvi, Rathri, Sarasvathi and Vac.The female deities in Rig Veda form the basis for the emergence of the later goddesses such as Shakthi, Devi and other mother goddesses glorified in the puranas. I think some of Rig Vedic deities are not dealt in puranas elaborately.
In the Rig Veda the goddess Usha is consistently associated with and often identified with the dawn. She reveals herself in the daily coming of light to the world. She has been described in the Rig Veda as a young maiden drawn by one hundred horses. She brings forth light and is followed by the sun who urges her onwards. She is praised for driving away, or is petitioned to drive away, the oppressive darkness. She is asked to chase away evil demons. As the dawn she is said to rouse all life, to set all things in motion and to send people off to do their duties. She sends the curled-up sleepers on their way to offer their sacrifices and thus render service to the other gods.
Another goddess commonly referred to in the Rig Veda is the goddess Prithvi who is nearly always associated with the earth, the terrestrial sphere where human beings live. In the Rig Veda furthermore she is always coupled with Dyaus the male deity associated with the sky. So dependent are these two deities in the Rig Veda that Prithvi is rarely addressed alone but almost always as part of the dual compound Dyaus-Prithvi, Sky-Earth. Together they are said to kiss the centre of the world. They sanctify each other in their complementary relationship. Together they are said to be the universal parents who created the world and the gods. As might be expected, Dyaus is often called 'father' and Prithvi 'mother'.