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They had the bodies, hind legs and tails of horses, but the front legs, wings and heads of what seemed to be giant eagles, with cruel, steel-coloured beaks and large, brilliantly orange eyes.
I don't know why 'but' is used in this sentence. It seems to me that it could be either a conj. or prep. What's 'but' doing there? Will it change the meaning of the sentence?
-- From Harry Potter.
I agree with Michael Rybkin. I think that "but" is used as conjuction there.
At least according to the meaning established in the Oxford Dictionary
but [conjuction]
Used to introduce a phrase or clause contrasting with what has already been mentioned.
‘he stumbled but didn't fall’
‘this is one principle, but it is not the only one’
‘the food is cheap but delicious’
‘the problem is not that they are cutting down trees, but that they are doing it in a predatory way’
The state of being strikingly different from something else in juxtaposition or close association.
Some parts [horse-like] are strikingly different from the other parts [eagle-like].
I have highlighted an specific example above where you can observe that the verb is used only once, the ellipsis mentioned by @Tᴚoɯɐuo
The food is cheap but delicious.
The food is cheap but (it's) delicious.
You got the same case in your example. Let's simplify
They had horse-like bodies but eagle-like heads.
They had horse-like bodies but (they had) eagle-like heads.
It had the head of a man but (had) the body of a lion.
It is a kind of ellipsis, I think. Perhaps there is a narrower technical term for it. The verb had is understood to govern both objects, the head of a man and the body of a lion.
So that but there joins two clauses in one of which the verb is merely understood to be present.
Do you think you will better understand it if I restructure your quote like this:
Question: They had the bodies, hind legs and tails of what?
Answer: Of horses.
Question: But what did they have the front legs, wings and heads of?
Answer: Of what seemed to be giant eagles, with cruel, steel-coloured beaks and large, brilliantly orange eyes.
Thus, technically speaking, but as used in your passage is a conjunction. It's making a contrast between different body parts that the creatures they're talking about have. The body parts that are listed at the beginning are those of horses, but the body parts which are listed later are like those that giant eagles have.
