Reptil, Dinomorpher

The design conceit here is escalation you buy in installments rather than at the point of casting. For a single green mana you get a 1/2 that threatens nothing on its own, but the two named activations are tiered power spikes: three mana turns it into a defensive 3/5 with vigilance and reach, six mana into a 6/6 trampler. Both last only until end of turn and both set the base stats rather than stacking, so this is a mana sink, not a permanent that grows. The Brontosaurus mode is the subtler one: vigilance and reach on a 3/5 lets a one-drop patrol the ground and the air while still swinging, a lot of coverage from a card you never had to hold up early. The Tyrannosaurus Rex mode is the finisher, converting surplus lands into trample damage. What ties it together is a single shifting body: you are always spending mana to inflate the same 1/2 for a turn, picking whichever dinosaur the situation calls for. That gives it a quiet resistance to removal keyed to power or toughness: a sorcery-speed answer hunting for something oversized to point at finds only a 1/2 until you commit the mana in response, and even then the swelling evaporates at end of turn.