As for Miek (who doesn’t have an actor since he doesn’t speak), he hasn’t been shown yet in the Thor: Ragnarok previews, but he’s been described as being a “larva-like creature” who has cybernetic limbs.
Both Korg and Miek made their comics debut in Planet Hulk, and since the Sakaar portions of Thor: Ragnarok are partially adapted from that storyline, it’s appropriate that they would be included in the movie.
Like his comic book counterpart, Korg is a Kronan, and he is being performed by director Taika Waititi.
These two aliens are quite lucky that they’ll be back for MCU installments, since most who are forced to fight on Sakaar are struck down in battle.