Thundermaw Hellkite

A 5/5 with flying and haste was already a known quantity before this one arrived: a hasty closer that swings for a fifth of an opponent's life the moment it lands. What separates it from the long line of five-mana red dragons that came before is the arrival clause that disarms its own best answer. By dealing 1 damage to and tapping every flyer your opponents control on entry, it sweeps the small defensive flyers (the spirits, the birds, the chumps) and freezes the larger ones for a turn, clearing the way for that first attack. That is the design that earned it a place where lesser hasty fatties stalled out: red's finishers have always had trouble punching through a wall of evasive blockers, and this one resolves the problem on the same turn it shows up. The damage is small enough that it rarely sweeps a board outright, which is the restraint that keeps it a closer rather than a removal spell stapled to a body. The whole point is that the clock starts ticking the instant it hits the battlefield, with no window for the opponent to set up a chump or a flying trade. A finisher built to actually finish, which is a narrower brief than it sounds and one that red's five-drops had repeatedly failed to fill.