A snake who lived in the Khandava forest. He was Thakshaka’s son, as per Mahabharata Adiparva Chapter 239 verse 5.

Once, when Krishna and Arjuna were staying on the banks of the Yamuna in summer, Agni requested them to help him consume the Khandava forest. Arjuna agreed to help him, and Agni gave him the Gandiva,  a white horse and several divine weapons. When Agni started consuming the forest, it began raining, at Indra’s command. Arjuna made a layer of arrows to block the rain. Ashwasena tried to escape but was blocked by Arjuna’s arrow. Ashwasena’s mother swallowed him and helped him escape the fire, but Arjuna’s arrrow beheaded her. Indra then tried to save Ashwasena by sending a fresh burst of wind and rains. Arjuna fell unconscious in that onslaught and Ashwasena managed to escape. When Arjuna awoke and couldn’t find Ashwasena, he cursed that the snake would not find shelter anywhere, as per Mahabharata Adiparva Chapter 226.

Ashwasena harboured a grudge against Arjuna for this. During the Mahabharata war, when Arjuna fought Karna, and Karna sent the nagastra against Arjuna, Aswasena hid in it and burned Arjuna’s crown, but when the arrow returned, Karna refused to accept it. Ashwasena was further incensed and attacked Arjuna, but was killed as per Mahabharata Karnaparva Chapter 96.