In George's defense, at some time in the early 90's this man thought to himself, I want to see ice zombies attacking a gigantic medieval fortress with dragons overhead. I'll always love him for that.

Sure, GRRM can't have it both ways. He's specifically mentioned the Mongols, Huns, the great plains nations, all as inspirations because it's a good talking point to sell the pseudo-authenticity of the books, but was very lazy at best in his depictions because he envisioned them functioning as the trope of "barbarians" without actually putting much thought into what that reveals about his own perspective. He's tried to smooth it over with the "mixed with fantasy" qualifiers, but that's pretty weak. Love the books and the show both, but as far as history goes, it's all much more telling about the stereotypes held by the western mind than anything real.

Doesn't really pertain to where he took the inspiration from, but sure, they're basically a faceless deus ex machina in the show, not much different from the green, scrubbing power of the Army of the Undead in the LOTR films. The ASOIAF books have a lot more nuance, but we'll see how that plays out if he ever finishes the final two.

GRRM based the Dothraki on the Mongols and other nomadic horse cultures, but yeah similar fighters, similar lifestyle.