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I've said it before I'll say it again.
Fire/Steel or Fire/Rock
Make it look like a full-on Shisa.
Give it "Spirit Guard" or some random name. Effect is Fairy-types can't switch out and Dark-types can't switch in unless they're all that's left.
Then just let it learn Iron Head again and maybe Head Smash if you go the Fire/Rock route. +20 to all stats, bam, Mega Arcanine. It's got nice stats, good movepool, the ever lovely Intimidate into Mega Ability combo, and most importantly it's thematically relevant. Just give it a design that's just ever so slightly over the top and it'll be perfect.