This is a <i>Jackson's Chameleon</i> and the little thing on its horns is actually its baby (this is the dad). It was born at the Taronga Zoo in Sydney, a few days ago.
The Jackson's is a real swinger, apparently. According to one source: "Male Jackson's Chameleons are always ready to mate and show this by putting on a spectacular display for every female they encounter. They flash brilliant colors, inflate their bodies to show their profile while rocking back and forth or head bobbing. Females show their acceptance by displaying a plain green or greenish grey coloration noticing the male chameleons display and indication that they are willing to mate. Non receptive females show their stress colors with grey and black blotching."
Yeah, I've gotten my share of grey and black blotching in my day.