Since the actual specifics of how legendary fish work has been in the dark for the vast majority of the community since they were added, I decided to finally write an explanation of how they all work.
All data I present here was collected by me and @Rickness_Voidwalker.
All legendary fish have a set spawn pattern based on time that can be tracked 100%, but they all also have weather requirements, which to my knowledge cannot be tracked. Blimp and Kylakin also aren’t too fond of flounder.
Non time based requirements are as follows:
Blimp: Requires there to not be raining and no flounder currently active in that pond, there is one exception which is the lower highsteppe pond, there you can catch blimp and flounder at the same time.
Kylakin: No rain and no flounder currently active in that pond.
Frosted Perch: requires rain.
Jelmiry: requires rain.
Da Majestic Flounder: requires the sky to be clear, aka no/very few coulds.
Global fish rotation
There are 5 timeslots that all fish rotations fall within, in UTC time they are:
00:00 - 05:00, 05:00 - 10:00, 10:00 - 15:00, 15:00 - 20:00 and 20:00 - 00:00
Blimp, Kylakin, Frosted Perch and Jelmiry all follow the same pattern, are active for the entirety of a timeslot at a time and are active the same every day, of course you also need to fill the weather/flounder requirements for them to be catchable.
Here’s most blimp rotations as an example:
So when testing for rotations you only need to check 5 times a day(not every 15 minutes for hours on end like we did before we realized the pattern )
Da Majestic Flounder
The Flounder is a bit more complex than the rest, it follows a 5 day cycle. When it starts it’s active for 1 hour and 15 minutes, then it’ll appear 1 hour later every day for 5 days, then it’ll go back to the same time as day 1 and repeat.
However, it also follows the 5 timeslots as the rest of the fish, this means that the first spawn of the 5 day rotation starts a little late for some rotations and the last spawn of other rotations are cut a little short. For the 4 hour timeslot there are a few flounder rotations that has it’s last spawn time cut of entirely.
Here’s a couple rotations so you can get a better idea of what I mean:
(The marked boxes are just start times)
Flounder can also have multiple rotations in the same pond, which I have so far not found in any other legendary fish.
All the rotations I know can be found in this spreadsheet: