This is Hayagriva, a minor avatar of Vishnu. Hayagriva, with horse head, is one of the three major deities of learning. The other two are Saraswati and Dakshninamurthy.
Vedas, by some accounts, are personified as human forms with horse heads. Vishnu as the presiding deity of Vedic learning is depicted with a horse head.
Here Hayagriva is seated in Padmasana. His lower right hand is in cinmudra, expounding knowledge gesture. He is holding Shankha, Akshamala and Pustaka is his hands, starting from upper right hand clockwise. Prescribed attributes of Hayagriva differs. The set of attributes, especially Akshamala as in this example, became prevalent after Madvacharya.
This idol is from Tamil Nadu or border areas (metal composition, inscribed base and casting style) and it is about 7 cm in height. The idol shows significant ritual wear. This may not date earlier than the seventeenth century.
Hayagriva is possibly the only Hindu God where a dish, Hayagriva Maddi, is named after God.