As Part Veela, Why Could Fleur Delacour Use Polyjuice Potion to Transform into Harry, and Why Could Hagrid Not?

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.}

Answer

First of all, I think you are slightly confused about directions:

• 
  

  Fleur able to take Polyjuice Potion and transform into Harry as you said

  
  


• 
  

  Whereas, Lupin testing Harry and NOT testing Hagrid meant that he was not worried that Hagrid was a fake Polyjuiced DE impostor.

  
  

In other words, what Lupin said was that it is not possible was for a human to transform into a half-giant (or non-human) [remember what happened to Hermione when she Polyjuiced herself into a cat in CS?] - but that didn't imply anything about a cross-breed transforming into human as a target. Thus, Fleur's transform into Harry doesn't necessarily make for a contradiction.

The limitation might only be on the target as opposed to the drinker of the potion.

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Leaving aside that obvious explanation, there are 4 options possible based on interpreting canon even assuming that someone could Polyjuice into Fleur (but not Hagrid):

1. 
   

   Polyjuice is (unlike Transfiguration) somewhat mass- or volume- dependent. While having different sized humans transform into Harry may be plausible, having a 1000 lb Hagrid who was twice human height is stretchin the laws of science magic - ditto transforming a human into Hagrid.

   
   


2. 
   

   This is most likely due to Hagrid being part-giant than the proportions. Giants in general seem immune to magic (see the scenes in HBP for example); and therefore it's quite possible that Polyjuice simply doesn't work well on them, same as any other magic.

   
   
   

   One of the reasons stated in canon for Hagrid's magic resistance is general thick-skinness and toughness of giants physically. It's possible that this purely biological physical distinction is problematic for Polyjuice transformation.

   
   

   Heck, Hagrid's hair may simply refuse to be dissolved in the potion, being too tough.

   
   


3. 
   

   Another possibility is that Giants - while technically the same species due to there being possible cross-offspring - are too genetically different from humans compared to Veela. Remember what happened to Hermione when she Polyjuiced herself into a cat in CS - that was likely due to genetic conflicts.