Sakas are described in Sanskrit sources as a Mleccha tribe grouped along with the Yavanas, Tusharas and Barbaras. There were a group of Sakas called Apa Sakas meaning water dwelling Sakas, probably living around some lake in central Asian steppes. Sakas took part in Kurukshetra War.
References in Mahabharata
The region called Sakadwipa
Mahabharata mentions about a whole region inhabited by Sakas called Sakadwipa to the north-west of ancient India. There in that region are, many delightful provinces where Siva is worshipped, and thither repair the Siddhas, the Charanas, and the Devas. The people there are virtuous, and all the four orders are devoted to their respective occupations. No instance of theft can be seen there. Freed from decrepitude and death and gifted with long life, the people there grow like rivers during the season of rains. The rivers there are full of sacred water, and Ganga herself, distributed as she hath been into various currents, is there as Sukumari, and Kumari, and Seta, and Keveraka, and Mahanadi and the river Manijala, and Chakshus, and the river Vardhanika, these and many other rivers by thousands and hundreds, all full of sacred water, are there. It is impossible to recount the names and lengths of rivers. As heard by all men there, in that region of Saka, are four sacred provinces. They are the Mrigas, the Masakas, the Manasas, and the Mandagas.
The Mrigas for the most part are Brahmanas devoted to the occupations of their order. Amongst the Masakas are virtuous Kshatriyas. The Manasas live by following the duties of the Vaishya order. Having every wish of theirs gratified, they are also brave and firmly devoted to virtue and profit. The Mandagas are all brave Shudras of virtuous behaviour.
In these provinces, there is no king, no punishment, no person that deserves to be punished. Conversant with the dictates of duty they are all engaged in the practice of their respective duties and protect one another. This much is capable of being said of the region called Saka.
The region called Sakadwipa is mentioned again at (12:14) as a region to the east of the great Karnali mountains.
The tale of Kamadhenu's army
When the sage Vasistha was attacked by king Viswamitra's army, Vasistha's cow, Kamadehnu, brought forth from her tail, an army of Palhavas, and from her udders, an army of Dravidas and Sakas; and from her womb, an army of Yavanas, and from her dung, an army of Savaras; and from her urine, an army of Kanchis; and from her sides, an army of Savaras. And from the froth of her mouth came out hosts of Paundras and Kiratas, Yavanas and Sinhalas, and the barbarous tribes of Khasas and Chivukas and Pulindas and Chinas and Hunas with Keralas, and numerous other Mlechchhas.
In the ancient Indian literature, cow is a symbol of earth or land. Thus the myth mentioned above simply means that, these tribes gathered for the protection of sage Vasistha's land against the army of king Viswamitra. This myth indicates that the ancient people were familiar with these Mlechcha tribes who were highly skilled in weapons, warfare and material sciences but never followed the Vedic rites properly.
The confusion of ancient Vedic people in dealing with these tribes is evident in the following passage from Mahabharata. At (12:35) is mentioned:- What duties should be performed by the Yavanas, the Kiratas, the Gandharvas, the Chinas, the Savaras, the Barbaras, the Sakas, the Tusharas, the Kankas, the Pathavas, the Andhras, the Madrakas, the Paundras, the Pulindas, the Ramathas, the Kamvojas, the several castes that have sprung up from Brahmanas and Kshatriyas, the Vaishyas, and the Shudras, that reside in the dominions of (Arya) kings?.
They were later given the status of Shudras. It is in consequence of the absence of Brahmanas from among them that the Sakas, the Yavanas, the Kamvojas and other Kshatriya tribes have become fallen and degraded into the status of Shudras. The Dravidas, the Kalingas, the Pulandas, the Usinaras, the Kolisarpas, the Mahishakas and other Kshatriyas, have, in consequence of the absence of Brahmanas from among their midst, become degraded into Shudras (13:33).
Encounters with the Sakas
Nakula the son of Pandu reduced to subjection the fierce Mlechchas residing on the sea coast, as also the wild tribes of the Palhavas, the Kiratas, the Yavanas, and the Sakas (2:31).
They were also vanquished by Krishna:- The Sakas, and the Yavanas with followers, were all vanquished by Krishna. (7:11).