Those especially tasty animals were created in the time the FailBoat was sinking. Sicpuess had the probably buzz-induced idea to catch some fish into an already submerged cage, to not have them catch them one by one later on, for they were tasty already.
Later on, with not much brain in his head, he decided to try if they would like zombie skin and catnip seeds as decoration. He maybe thought that they could use some water plants, but no, the fish had to eat it all. Since zombie is an excellent fertilizer, the seeds grew and merged with the fish. What then came into being, were fishes with their scales replaced by little catnip leaves.
The feet-long fish are a popular dish especially amongst Kittymorphs, but also amongst other races, of course then not raw, mostly. Therefore, more cages were occupied, thus forming the catnipfish hydroculture on the FailBoat. Since it sank, the construction floats above the place in form of a raft.
On the second try on putting some of the fish into the koi pond in Common Ground1), it actually worked.
For a short time, it went well... then a glass of Joker Special contaminated the water, throwing the fish into speed evolution2). They increased quickly in intelligence, and began to collect and try out different mind patterns, eventually forming kind of swarm intelligences, which, after merging to one single and in search of more information, repeatedly tried to take over their creator3).
The IBHML decided to investigate - a conference in Improbable Central came to the result that the fish were collecting language patterns, allowing their carriers to speak and understand various languages - but not to actually control which language is spoken.
In the end, due to the discovery of the other, main intent of the swarm4), the carriers discarded their fish and sealed them into an Improbable Warp. The remaining swarm wasn't very happy, and hit them over the head with mindwaves, leaving only confusion and headaches. A fish heavily intoxicated with chrome, thus becoming a googlefish, took over Anonymous Person, but was squished afterwards.
Eventually, the whole fish part of the koi pond was sealed into the same warp, and the pond water was cleaned. Unaltered catnipfish inhabitate the pond by now, living in happy coexistence with the other habitants.
Recently, the sealed warp was opened - due to the memory that the water in there was still contaminated with Joker Special - and Sicpuess was hit once again, but eventuall, the warp closed itself, and stayed so. Further information is classified uncorroborated, and will only be published if status changes.