Aṃśumāna (Angshumana) was one of the many kings present at Draupadī’s (Draupadi’s) Svayamvara (Swayamvara) ceremony. However, not much information is available on his lineage in Mahābhārata (Mahabharata). In the Karṇaparva (Karnaparba) phase of the Kurukṣetra (Kurukshetra) war, Sañjaya lists the names of warriors killed so far. Among those names we find the name of Bhoja king Aṃśumāna, who fought alongside the Pāṇdavas (Pandavas) and was killed by Droṇa. Harivaṃśa (Harivamsha) provides more information on the lineage of this King Aṃśumāna; he is said to have hailed from the Krutha clan. Till the time of Rukmiṇī’s (Rukmini) abduction, he was a resident of Vidarbha, where he remained subservient to Bhīṣmaka (Bhishmaka) out of fear. But it appears from the chain of events in Mahābhārata that at a later period, Aṃśumāna supported Kṛṣṇa (Krishna), and following this connection eventually joined the Pāṇdava (Pandava) camp.
A detailed study of Bhāgavatapurāṇa (Bhagabatapurana) tells us of a certain Aṃśumāna who was put in charge of the sacrificial horse during Kṛṣṇa’s Aśvamedha (Ashwamedha) yajña (yajna). It appears that the Aṃśumāna referred to here, and the one mentioned in Mahābhārata and Harivbaṃśa are one and the same.
Ref. verse 3 of footnote of Bhāgavatapurāṇa (ed. Krishna Sankar Shastri). 10.89.22.