The most accessible format! In Pauper, all cards in your deck must have been released at common rarity in a Magic set or product.
Read more at Pauper's official website
It's Cranial Ram if you haven't seen it yet
This is perhaps the poster-boy for an uncommon to me. Not so strong to make it a rare, and definitely way too much complexity to make it a common. No clue on earth why they would print something like this at common. I think generally if you're printing "living weapon" on something it needs to start at uncommon but I see there are some other cards already at common with that ability.
It's worth mentioning that at the time Cranial Plating was first banned, Ornithopter was not legal in Pauper (it was on MTGO but only at uncommon). As Alex says, [[Somber Hoverguard]] was the best Plating-bearer, and you couldn't realistically be swinging with it sooner than turn 4, although you could easily hit for a third or more of the opponent's life total when you did. Now Ornithopter is legal and you could do the equivalent with a Cranial Ram on turn 3.
I think between the mana cost and the equip cost(s), Ram is slower than Plating by enough that it's worth a trial. I won't be shocked or sorry if it gets banned, but I also won't be surprised if it turns out to be safe.