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In Book 5, when Hagrid visits the giants, he is told that his mother had died years ago. Perhaps those giants were lying and his mother is with another group of giants, and she is their queen. The group that Hagrid visited were mistreating his half-brother Grawp not just because he was scrawny, but because he was a hostage.

When Hagrid and Grawp finally break through the commucation barrier, Grawp tells Hagrid where their mother is, they go find her, and her group of giants is willing to help in the fight against Voldemort.

That makes Hagrid the Half-Blood Prince of the giants.