You're unable to play Garruk Relentless in GW because his flipside has a Black color-identity
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I get what you mean, the meta always defines what to use and what not to use, especially in this format.
With any form of a sac outlet as well as Butcher/Pact, Artist is always a good choice
Thing about the centaurs though, is I wouldn't cast Horror unless I had clear control of the board, or if I had any of my ways to deal with them out i.e. Mystifying Maze, any of my non-black creatures, Decree of Pain in Hand, a plague boiler ready to blow, or my Rogue's Passage and a big enough creature. With Horror, you have to be a teensy bit conservative in the way you play him, but he's just as good as Troll if used properly.
Viscera Seer is a good utility one-drop who can feed into Butcher and Gravepact. Experiment One is another good one, as he's more likely than not gonna stick around.
This is just me, but Slitherhead just doesn't do enough to be productive, and same with Force of Savagery.I might suggest (as you saw from my list) either or both of the Hunted Creatures, Kokusho the Evening Star and Plague Boiler as options in their places.
Yeah, the Hunted Creatures are great. Especially in Varolz, since the Horror is 7 Counters for just BBAnd Plague Boiler is a great clock that can go off in response to removal (sans Krosan Grip) if enough mana is available.