Mistform Wall

The clever knot here is in the templating text: "This creature has defender as long as it's a Wall." Change its type away from Wall with the activation, and the defender clause stops applying, so the 1/4 can swing. That conditional is the entire reason the card exists, a Wall that turns off its own Wall-ness to attack, then can be sent back to blocking duty next turn since the type change wears off at end of turn. The four toughness makes the defensive mode genuinely sticky against the small attackers it was meant to wall off, while the one-mana type shift doubles as Onslaught-era tribal glue: any tribal payoff keyed to a creature type can name this as a member for a turn, and any attack you want it to make is one activation away. Most of the Mistform cycle traffics in the same shapeshifter conceit, but this one is the only piece built around the defender interaction specifically, using a rules quirk (a granted keyword that hinges on a type the card can shed) as its load-bearing mechanic rather than a flavor flourish. It is a defensive body that can choose, for a turn at a time, not to be defensive at all.