It would be an error to interpret ancient Indian history using the black and white caste stereotypes of 19th-century colonial India.
Also, it is important to realize that dalits are not part of society in Hinduism. They are outcasts or foreigners. In proper and true sacred Hinduism there is no "upper" or "lower", there are only four roles: priest, warrior, trader, worker. Each of these is divided into different sub-roles. No one role is any closer to Brahma than any other role. A holy Shudra can be closer to Brahma than a polluted Ksatriya. Simply being in one role or another does not inherently make you closer to Brahma.
The dalits are those, like foreigners, who are outside the system altogether and do not even know Brahma at all. They are not part of "society" in the Hindu view, they are outside of it.
To answer your specific question, "who were they?" They were not anyone. Closeness to Brahma is not defined by family or ethnic group. The idea that certain families have "caste" or such is a false British stereotype that has no place in real Hinduism. A dalit is not defined by being a member of a certain ethnic group or people. It is anyone who does not know Brahma at all. So, that includes all foreigners or anyone in India who for whatever reason rejects or remains ignorant of Brahma. If a Brahmin priest rejects Brahma or pollutes himself or becomes ignorant of Brahma by madness or for any other reason, he becomes dalit: an outcast. The fact that he may have been originally born into a Brahmin family is completely irrelevant. It is his karma that determines whether he is dalit or not.