Dan dropped onto all fours, the weight he was putting on made it impossible to stand on two legs. His skin became darker and thicker. His spine lengthened as he became much, much larger. The floor creaked under his weight, his eyes, nose and ears all migrated to the top of his skull which itself became much larger and longer.
Dan realized that he had become a hippopotamus. Strange, he had never felt like the 'animal inside him' was a hippo. He was then struck how monumentally stupid his sister had been. Hippos put lions, alligators, and crocodiles to shame. Hippos were the most deadly animals in all of Africa. Dan waited.
Soon enough Kat showed up, her eyes opened wide, and Dan rammed her with his head. The effect was better than anything he could have imagined. Her wand shattered into a thousand pieces and, he was pretty sure, zapped her in the process. She didn't fair well herself; she was knocked out of Dan's second story window. Based on the moans he heard Dan guessed that she was realtively unharmed, but wasn't sure that she was still human.