When the son of Mahārāja Nābhi became visible, He evinced all good qualities described by the great poets — namely, a well-built body with all the symptoms of the Godhead, prowess, strength, beauty, name, fame, influence and enthusiasm. When the father, Mahārāja Nābhi, saw all these qualities, he thought his son to be the best of human beings or the supreme being. Therefore he gave Him the name Ṛṣabha.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
तस्य ह वा इत्थं वर्ष्मणा वरीयसा बृहच्छ्लोकेन चौजसा बलेन श्रिया यशसा वीर्यशौर्याभ्यां च पिता ऋषभ इतीदं नाम चकार
tasya ha vā itthaṃ varṣmaṇā varīyasā bṛhacchlokena caujasā balena śriyā yaśasā vīryaśauryābhyāṃ ca pitā ṛṣabha itīdaṃ nāma cakāra
SB 5.4.2