"Well, I've bin doin' some readin'," said Hagrid, pulling a large book from under his pillow. "Got this outta the library -- Dragon Breeding for Pleasure and Profit -- It's a bit outta date, o' course, but it's all in here. Keep the egg in the fire, 'cause their mothers breathe on 'em, see, an' when it hatches, feed it on a bucket o' brandy mixed with chicken blood every half hour. An' see here -- how ter recognize diff'rent eggs -- what I got there's a Norwegian Ridgeback. They're rare, them."
What's the sentence "They're rare, them" conveying? I can figure out 'they' refers to "Norwegian Ridgeback eggs", but I don't understand what 'them' is doing there? I feel the structure of the sentence is unusual, so I'm not sure if this kind of sentence construction has some special effect. Is it a part of standard English structures, which I'm not yet familiar with?
-- Excerpted from Harry Potter.