Just because something is forbidden doesn't mean you can't get it. These ferocious pets are available for purchase in Camden Passage. Be sure to ask for Seth, and inquire about the "meat pies."
Billyfish were once abundant in London’s rivers. These magic sensitive fish can home in on spells and chase after the magicians that cast them. They can survive out of water for long periods of time. A real nuisance.
There would be schools of them flipping and flopping about the streets, banging against people's doors and sneaking into their houses. Normal people started to catch on and it became dangerous for us magicians. Using a Billyfish became a way to sniff us out." — Mr. Maddox, owner of the Extraordinary Curiosities of Ixworth and Maddox