Hearthpwn

While I agree it seems like a card that could fit nicely into the zoolock deck theme, the reality is that its too sloe of a card and isn't reliable enough.

My understanding of the reason for Defender of Argus is not specifically the taunt, bit for the buff to make minions either just deal more damage, or force your opponent to make in-favourable trades.

Mukla's champion is a bit too slow because by turn 5 you want to be pushing for evendors more damage, so you'll want a card that has a more direct impact on the board or their face. Not to mention that replacing some low cost cards with 5 mana ones makes the deck less consistent, you're less likely to pull cheap minions on those first couple turns which you so desperately need.

Mukla's Champion falls into the "win more" catagory of cards, where it's best when you have a winning board state.