Who/what does Kipling mean when he refers to "Your new-caught Sullen peoples,/half-devil and half-child"?

Joseph Rudyard Kipling was an English poet, novelist, and journalist. He was born in 1865 and died in 1936.

Despite having born in India and have been raised there for a part of his life, Kipling agreed to racist and imperialist ideologies that believed that the white man, the white civilizations such as Britain and the US, had the "burden" of civilizing and educating the supposedly less developed societies of the world.

The verses Your new-caught Sullen peoples,/half-devil and half-child belong to his famous poem The white man's burden published in 1899 at the height of the modern imperialism of 19th and 20th centuries.

The verses refer to African and Asian peoples from territories that were being colonized for the economic growth of white civilizations. That's why Kipling talks about "new-caught", because of recently colonized societies, and "half-devil and half-child" as these peoples were believed to be stupid and immature.