The only one I can think of off the top of my head is Steelix prime but it costs a double colorless an is a stage 2. Are you also talking strictly discard or from anywhere (deck/Bench/hand.)
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Yeah I definitely see Terrakion as a solid counter against Zekrom and Magnezone but it's too much like Bouffalant to get that extra damage you need for the KO. It's best used when your Opponent has taken a prize on their last turn and then you can Catcher their Zekrom or Magnezone for a KO. With Eviolite though Zekrom might be hard to take down however against Zone it has a chance.