Rishika was an ancient kingdom inhabited by the Rishikas who were originally a tribe north to Himalayas who had limited interaction with early Indian kingdoms and mentioned in the epic Mahabharata. They belonged to the Xinjiang province of China, east of Kashmir. The Pandava hero, Arjuna visited this place during his northern military campaign for collecting tribute for Yudhishthira's Rajasuya sacrifice (Lohan.paramaKambojanRishikan uttaranapi).
References
Rishika Kingdom Wikipedia(Text) CC BY-SA