It turns out cassowaries have been domesticated—as guard-fowl.
I figure some brave soul grabbed one of the GIANT LUMINOUS NEON MARKER GREEN eggs (the colour of "do not fuck with me for I am deadly" in nature) and ran away with it. Then it hatched, and they fed the giant murder chicken that kept following them around.
<supervillain>
When I get my island volcano base the inland jungle will be patrolled by giant murder chickens LOYAL ONLY TO ME! Take that, laser-sharks!
</supervillain>
@cstross Yay for pet dinosaurs!