What are the origins of things such as giants, goblins, and rieklings? Do they have any relation to humans and elves? I've heard that giants came from Atmora and the goblins are degenerated elves or something. How long have they been around?

That is a very good question. I know Malacath claims dominion over ogres, and in fantasy ogres, orcs, and goblins are often lumped together. Maybe from him?
I'd be very curious to see this explored (and with a Kirkbridian "Vivec farted them out so Talos would have someone to play ping pong with" answer)