Iron Mastiff

The 4/4 body is the boring part; the attack trigger is a d20 table bolted to the declare-attackers step, adding a burst of damage on top of the creature's ordinary combat damage rather than replacing it. Swing, roll, and let the number rule: a 1 through 9 sends four back at its own controller, a 10 through 19 hits the defending player, and a natural 20 sprays four at every opponent at once. All of that resolves as the mastiff enters combat, so the trigger and the eventual combat damage are two separate events; a clean roll lands the trigger damage before the dog then bites again in the combat damage step. The multiple-die clause rewards spreading an assault rather than piling onto one player: you roll one d20 per player being attacked and keep only the highest, so pointing several attackers at several different players in the same combat stacks the odds of clearing the 10 threshold far higher than a lone swing manages. This wires the die-roll subtheme into the attack step, where its variance carries a self-directed downside that spell-based rolls rarely do: the same swing can advance your board, whiff, or turn around on you. The cost is control over the outcome, and it applies only on the offensive. The trigger never fires on defense, so a mastiff held back is simply a vanilla 4/4 wall with none of the gamble.