Is caste system only for humans? What about animals, Asuras, Devas etc. Does caste system also apply to them?
Brahman initially projected Varna/cast in deities, Varna of deities as described in Brihadaranyaka:
Kshatriya- Indra, Varuna, the moon, Rudra, Parjanya, Yama, Death, and Isana
Vaishya- Vasus, Rudras, Adityas, Visvadevas and Maruts
I-iv-11: In the beginning this (the Kshatriya and other castes) was indeed Brahman, one only. Being one, he did not flourish. He specially projected an excellent form, the Kshatriya - those who are Kshatriyas among the gods: Indra, Varuna, the moon, Rudra, Parjanya, Yama, Death, and Isana. Therefore there is none higher than the Kshatriya. Hence the Brahmana worships the Kshatriya from a lower position in the Rajasuya sacrifice. He imparts that glory to the Kshatriya. The Brahmana is the source of the Kshatriya. Therefore, although the king attains supremacy (in the sacrifice), at the end of it he resorts to the Brahmana, his source. He who slights the Brahmana, strikes at his own source. He becomes more wicked, as one is by slighting one's superior.
I-iv-12: Yet he did not flourish. He projected the Vaisya - those species of gods who are designated in groups: the Vasus, Rudras, Adityas, Visvadevas and Maruts.
I-iv-13: He did not still flourish. He projected the Sudra caste - Pusan. This (earth) is Pusan. For it nourishes all this that exists.
I-iv-15: (So) these (four castes were projected) - the Brahmana, Kshatriya, Vaisya and Sudra. He became a Brahmana among the gods as Fore, and among men as the Brahmana. (He became) a Kshatriya through the (divine) Kshatriyas, a Vaisya through the (divine) Vaisyas and a Sudra through the (divine) Sudra. Therefore people desire to attain the results of their rites among the gods through fire, and among men as the Brahmana. For Brahman was in these two forms. If, however, anybody departs from this world without realising his own world (the Self), It, being unknown, does not protect him - as the Vedas not studied, or any other work not undertaken (do not). Even if a man who does not know It as such performs a great many meritorious acts in the world, those acts of his are surely exhausted in the end. One should meditate only upon the world of the Self. He who meditates only upon the world called the Self never has his work exhausted. From this very Self he projects whatever he wants.
Caste system is there at least for the Planetary-Devatas or the Navagrahas,
- Castes of Grahas. Guru and Śukr are Brahmins. Sūrya and Mangal are royal Grahas, while Candr and Budh belong to commercial community. Śani rules the Sudras (4th caste).
Brihat Parashara Hora Shastra 3.21.
So, according to this classification, Brihaspati, Sukra are Brahmins. Surya, Mangal are Kshatriyas. Mercury (Budha) and Moon (Chandra) are Vaishyas whereas Sani is Sudra.
Some say that Rahu and Ketu belong to the Antyaja or the mixed caste (or out caste) category, but there are mantras which mention both of them as Sudras.
Given below are the starting portions of the Dhyana Mantras (Meditative verses) of some of the planetary deities. These mantras also mention their castes explicitly:
Surya- Om Kshatriyam Kashyapam Raktam Kalingam.....
Chandra- Om Samudram Vaishyamatreyam Hastamatram Sitamavaram...
Mangal- Om Avantyam Kshatriyam Raktam Meshastham Chaturangulam...
Brihaspati- Om Dvijamangirasam Pitam Saindhavancha Shadangulam....
Sani- Om Saurashtram Kashyapam Sudram Suryansyam Chaturangulam...
Rahu- Om Malayajam Sudram Paithinam Dvadashangulam....
As far as Daityas, Asuras, different kinds of animals etc are concerned, they can certainly be classified according to Gunas (Satva, Rajas, Tamas) but they may not have caste- categories.
Also, there is apparently a difference as far as caste system among humans and that among the Devatas are concerned.
For example, as shown in this answer, Ketu Graha, inspite of being a Sudra still wears the sacred thread. But for a human-sudra wearing it is not allowed.
raudrō rudrapriyō rudraḥ krūrakarmā sugandhadhrik |
He who is angry, he who likes Shiva, he who is fierce, He who does cruel deeds, He who is seen by pleasant smell, One who resembles the smoke of husk, one who is seen wearing sacred thread.