Warg Riders

[The Squatter of Amon Rûdh]

there is certainly a suggestion somewhere (couldn't find the quote) that this might in fact be their origin.

Perhaps you mean Treebeard's comments about Saruman:

He has taken up with foul folk, with the Orcs. Brm, hoom! Worse than that, he has been doing something to them; something dangerous. For these Isengarders are more like wicked Men. It is a mark of evil things that came in the Great Darkness that they cannot abide the Sun; but Saruman's Orcs can endure it, even if they hate it. I wonder what he has done? Are they Men he has ruined, or has he blended the races of Orcs and Men? That would be a black evil!

Or perhaps Gamling's comment to Aragorn at the Hornburg: "But these creatures of Isengard, these half-orcs and goblin-men that the foul craft of Saruman has bred, they will not quail at the sun".