In Harry Potter and the Half-Blood Prince, chapter 22 Hagrid says the following:
"Did he?" said Hagrid, looking both astonished and touched. "Tha's — tha's righ' nice of him, that is, an' not turnin' yeh in ei-ther. I've never really had a lot ter do with Horace Slughorn before. .. . Comin' ter see old Aragog off, though, eh? Well. . . he’d've liked that, Aragog would. . . ."
That sounds fair BUT ... Hagrid studied almost 3 years in Hogwarts and started 2 years after Voldemort. We know that Slughorn was a teacher at the the time when Voldemort was in Hogwarts.
That means that Slughorn also taught Hagrid. So it is not like he "did never really had a lot ter do with Horace Slughorn".
Why did Hagrid claim to not really have a lot to do with Slughorn?