Hagrid doesn't seem to be afraid of any animals or people. But in some sense he is timid.
Is there any indication what Hagrid's boggart form is?
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Within the main canon, we simply don't know.
Guesswork: Like Harry, he certainly isn't fond of Dementors and a very common boggart form among his contemporaries is Lord Voldemort. He also has several friends (Dumbledore, notably) and a close relative (Grawp) that he wouldn't want to see their corpses.
The Official Harry Potter Bloomsbury Website contains a quiz about "your Hogwarts best friend" in which we learn the main fears of various characters, including Hagrid.
Assuming these fears also represent the person's boggart form, Hagrid's would be Azkaban Prison.