Although he (Visvamitra) was a ksatriya by birth, he became a brahmana in the very same body by the power of his spiritual achievements
SB Canto 1
From different parts of the universe there arrived great sages like Atri, Cyavana, Śaradvān, Ariṣṭanemi, Bhṛgu, Vasiṣṭha, Parāśara, Viśvāmitra, Aṅgirā, Paraśurāma, Utathya, Indrapramada, Idhmavāhu, Medhātithi, Devala, Ārṣṭiṣeṇa, Bhāradvāja, Gautama, Pippalāda, Maitreya, Aurva, Kavaṣa, Kumbhayoni, Dvaipāyana and the great personality Nārada.
Cyavana: A great sage and one of the sons of Bhṛgu Muni. He was born prematurely when his pregnant mother was kidnapped. Cyavana is one of the six sons of his father.
Bhṛgu: When Brahmājī was performing a great sacrifice on behalf of Varuṇa, Maharṣi Bhṛgu was born from the sacrificial fire. He was a great sage, and his very dear wife was Pulomā. He could travel in space like Durvāsā, Nārada and others, and he used to visit all the planets of the universe. Before the Battle of Kurukṣetra, he tried to stop the battle. Sometimes he instructed Bhāradvāja Muni about astronomical evolution, and he is the author of the great Bhṛgu-saṁhitā, the great astrological calculation. He explained how air, fire, water and earth are generated from ether. He explained how the air in the stomach works and regulates the intestines. As a great philosopher, he logically established the eternity of the living entity (Mahābhārata). He was also a great anthropologist, and the theory of evolution was long ago explained by him. He was a scientific propounder of the four divisions and orders of human society known as the varṇāśrama institution. He converted the kṣatriya king Vītahavya into a brāhmaṇa.
Vasiṣṭha: See Śrīmad-Bhāgavatam 1.9.6.
Parāśara: He is the grandson of Vasiṣṭha Muni and father of Vyāsadeva. He is the son of Maharṣi Śakti, and his mother's name was Adṛśyatī. He was in the womb of his mother when she was only twelve years old. And from within the womb of his mother he learned the Vedas. His father was killed by a demon, Kalmāṣapāda, and to avenge this he wanted to annihilate the whole world. He was restrained, however, by his grandfather Vasiṣṭha. He then performed a Rākṣasa-killing yajña, but Maharṣi Pulastya restrained him. He begot Vyāsadeva, being attracted by Satyavatī, who was to become the wife of Mahārāja Śāntanu. By the blessings of Parāśara, Satyavatī became fragrant for miles. He was present also during the time of Bhīṣma's death. He was spiritual master of Mahārāja Janaka and a great devotee of Lord Śiva. He is the author of many Vedic scriptures and sociological directions.
Gādhi-suta, or Viśvāmitra: A great sage of austerity and mystic power. He is famous as Gādhi-suta because his father was Gādhi, a powerful king of the province of Kanyākubja (part of Uttara Pradesh). Although he was a kṣatriya by birth, he became a brāhmaṇa in the very same body by the power of his spiritual achievements. He picked a quarrel with Vasiṣṭha Muni when he was a kṣatriya king and performed a great sacrifice in cooperation with Maṭaṅga Muni and thus was able to vanquish the sons of Vasiṣṭha. He became a great yogī, and yet he failed to check his senses and thus was obliged to become the father of Śakuntalā, the beauty queen of world history. Once, when he was a kṣatriya king, he visited the hermitage of Vasiṣṭha Muni, and he was given a royal reception. Viśvāmitra wanted from Vasiṣṭha a cow named Nandinī, and the Muni refused to deliver it. Viśvāmitra stole the cow, and thus there was a quarrel between the sage and the King. Viśvāmitra was defeated by the spiritual strength of Vasiṣṭha, and thus the King decided to become a brāhmaṇa. Before becoming a brāhmaṇa he underwent severe austerity on the bank of the Kauśika. He was also one who tried to stop the Kurukṣetra war.
Aṅgirā: He is one of the six mental sons of Brahmā and the father of Bṛhaspati, the great learned priest of the demigods in the heavenly planets. He was born of the semen of Brahmājī given to a cinder of fire. Utathya and Saṁvarta are his sons. It is said that he is still performing austerity and chanting the holy name of the Lord at a place known as Alokānanda on the banks of the Ganges.
Paraśurāma: See Śrīmad-Bhāgavatam 1.9.6.
Utathya: One of the three sons of Maharṣi Aṅgirā. He was the spiritual master of Mahārāja Mandhātā. He married Bhadrā, the daughter of Soma (moon). Varuṇa kidnapped his wife Bhadrā, and to retaliate the offense of the god of water, he drank all the water of the world.
Medhātithi: An old sage of yore. An assembly member of the heavenly King Indradeva. His son was Kaṇva Muni, who brought up Śakuntalā in the forest. He was promoted to the heavenly planet by strictly following the principles of retired life (vānaprastha).
Devala: A great authority like Nārada Muni and Vyāsadeva. His good name is on the list of authorities mentioned in the Bhagavad-gītā when Arjuna acknowledged Lord Kṛṣṇa as the Supreme Personality of Godhead. He met Mahārāja Yudhiṣṭhira after the Battle of Kurukṣetra, and he was the elder brother of Dhaumya, the priest of the Pāṇḍava family. Like the kṣatriyas, he also allowed his daughter to select her own husband in a svayaṁvara meeting, and at that ceremony all the bachelor sons of the ṛṣis were invited. According to some, he is not Asita Devala.
Bhāradvāja: See Śrīmad-Bhāgavatam 1.9.6.
Gautama: One of the seven great sages of the universe. Śaradvān Gautama was one of his sons. Persons in the Gautama-gotra (dynasty) today are either his family descendants or in his disciplic succession. The brāhmaṇas who profess Gautama-gotra are generally family descendants, and the kṣatriyas and vaiśyas who profess Gautama-gotra are all in the line of his disciplic succession. He was the husband of the famous Ahalyā who turned into stone when Indradeva, the King of the heaven, molested her. Ahalyā was delivered by Lord Rāmacandra. Gautama was the grandfather of Kṛpācārya, one of the heroes of the Battle of Kurukṣetra.
Maitreya: A great ṛṣi of yore. He was spiritual master of Vidura and a great religious authority. He advised Dhṛtarāṣṭra to keep good relations with the Pāṇḍavas. Duryodhana disagreed and thus was cursed by him. He met Vyāsadeva and had religious discourses with him.