Niantic may hold an April Fools Event that features Magikarp spawn only day. An avid Pokemon GO player presented this theory and his supporting evidence.
Magikarp Spawn Only Day
Reddit user Dangles believes that Pokemon GO developer will hold a Magikarp only spawn day on April Fools. The theory seems far-fetched but he did present some supporting theory for this "foolish" idea.
In the past, the developer held a Pikachu medal event which allowed players to catch more of them. It also unleashed a lot of Ratata for the Tiny Medal. Dangles believed that Niantic wants to give Pokemon GO players a chance to earn their Fisherman Medals by releasing tons of Magikarp in the game. He also thinks that April Fools Event is the time this theory is likely to happen.
He also presented a table showing a number of Silph Road users who caught a lot of Magikarp but only a small percentage were eligible for the Fisherman Medal. It seems only very few players in the list were lucky enough to get one medal for every five Magikarp they caught. The others were not so lucky to get that much. The Fisherman medal is only awarded for big Magikarp.
What's So Special About Magikarp
Basically, Magikarp are the butt of jokes in Pokemon television shows and movies. If you evolved it to a Gyarados though, it's one of the tough Gen 1 Pokemon in gym battles.
Pokemon GO Events
Niantic has been holding several Pokemon GO events that have convinced a lot of casual players to go back. The recent two major ones are Halloween and Christmas. These events are notable for increasing the spawn of certain types of Pokemon. Niantic spawn a lot of ghost types during Halloween and Santa Pikachu during Christmas. Lapras were spawning in droves during a local event in Japan.