The Animal King first appears in the "Animal King Takeover" service announcement slideshow when the Announcer is about to give advice about what to do when Earth is being taken over by an "animal king, sentient cloud, or other governing body that either refuses to or is incapable of listening to reason", before the recording is cut short. In the slideshow, the Animal King, appearing as a gigantic Turret covered with a leopard skin-patterned texture and wearing a crown, drops into a United Nations amphitheater and opens, causing people to either flee or bow to it.
The Animal King appears again in the back of the amphitheater during the Turret Opera at the very end of the game, where it is shown to be playing the deeper bass tones of the song.
While the Animal King acts more as a symbol, the model itself was likely for sale to the general public. Indeed, a smaller version of the Animal King, minus the crown, can be seen in Aperture Investment Opportunity #3: "Turrets".
"Animal King" is a registered trademark of Aperture Science, copyrighted 1998.
Behind the scenes
According to Erik Wolpaw, at one point in Portal 2 Chell was to stumble upon a pile of abandoned turrets resembling a lost tribe one might find surviving on a forgotten island, and help them in some way. The Animal King would then marry off one of its turrets to her. It would follow Chell around for the rest of the game.