It is established in Deathly Hallows that Polyjuice Potion is meant for human transformations only:
‘Wha’ was tha’ about?’ roared Hagrid.
‘I’m sorry, Harry, but I had to check,’ said Lupin tersely. ‘We’ve been betrayed. Voldemort knew that you were being moved tonight and the only people who could have told him were directly involved in the plan. You might have been an impostor.’
‘So why aren’ you checkin’ me?’ panted Hagrid, still struggling to fit through the door.
‘You’re half-giant,’ said Lupin, looking up at Hagrid. ‘The Polyjuice Potion is designed for human use only.’
Deathly Hallows - page 63 - Bloomsbury - chapter 5, The Fallen Warrior
And it's established Fleur Delacour is part Veela in Goblet of Fire:
[Ollivander] twirled the wand between his long fingers like a baton and it emitted a number of pink and gold sparks. Then he held it close to his eyes and examined it carefully.
‘Yes,’ he said quietly, ‘nine and a half inches ... inflexible ... rosewood ... and containing ... dear me …’
‘An ’air from ze ’ead of a Veela,’ said Fleur. ‘One of my grandmuzzer’s.’
So Fleur was part Veela, thought Harry [.]
Goblet of Fire - page 270 - Bloomsbury - chapter 18, The Weighing of the Wands
Why was Fleur able to take Polyjuice Potion and transform into Harry while Hagrid was not? Neither Fleur nor Hagrid are fully human.
I'm not interested in an answer that says it was possible because Fleur had more human blood than Hagrid. It's clear from canon that "human use only" means human use only. Can this discrepancy be explained within the spirit of canon? I don't know of a canon explanation for this question, or that J.K. Rowling addresses it in any of her interviews, but if you can find a quote or passage from the books that answers this, I would love it if you left an answer. Please no HP Wiki/Wikia answers.
I checked for this question under "Fleur" and didn't find it. I hope it's not a dupe.