Pre-named fish are special and extremely rare fish that have their own color schemes, descriptions, and hometowns and usually have special abilities other fish don't have (such as Johnny V's floating text). They cost 50,000 shells, with the exception of Santa.
There are a total of six pre-named fish but only four of them can be purchased from the store slot containing attributed fish: Kilgore, Ludwig, Rocky, and Johnny V. Santa and Cookie can be purchased under their own unique circumstances. Renaming them after buying won't change their attitude.
Assuming January 1st of 2019 is day 1 and December 31st is day 365, fish that are pre-named are available from The Fish Emporium on days that begin with an odd number and end in 4, as in 14, 34, 54, and so on. Once all four pre-named fish have been bought, these days will have extremely rare fish instead.
Johnny V is a rainbow carnivore who can't stop eating pizza. He costs 50,000 shells. Every few times that Johnny is fed, some text will appear just above him reading, "This is my last pizza, I swear!"
Kilgore is a singing ultravore that is purple in color, and can roar as well. He costs 50,000 shells. He sometimes sings Ride of the Valkyries when hungry and will interrupt the song halfway through if he is fed. He only roars if fed while singing.
Ludwig is a blue and black carnivore with aqua fins. He costs 50,000 shells. Ludwig is a musical genius who sings various songs by Ludwig van Beethoven, including several extended versions.
Rocky is a blue guppy with white fins who eats ultravores as per doctor's orders. He costs 50,000 shells. Rocky roars when he eats. For some strange reason, Rocky may disappear from the player's tank. He drops bags o' shells but never grows.
Santa is a guppy that is acquired by naming a non-colored guppy in The Fish Emporium "Santa." It will be white with red fins. Santa sings Christmas songs and, when fully grown, it will drop Bags of Shells. Santa grows every time it's fed.