Though there certainly are many reasons for the popularity of the Pokemon franchise at large, it's the pocket monsters themselves that are the obvious stars of the show. With over two decades' worth of lore and backstory explaining certain species in great detail, there are still blanks that fans enjoy filling in, and artist Alex Fantastico, in particular, has much to share in this regard.
Namely, Fantastico has taken it upon themselves to create in-depth Pokemon evolutionary charts that track creatures' lineages in substantial detail. So far, they had created a number of unique charts featuring Pokemon's dogs, foxes, and other assorted creatures, but their latest endeavor is dedicated to otter and otter-adjacent Pokemon, specifically.
Much as was the case with Fantastico's fox Pokemon ancestry chart, here to the artist needed to come up with a few intermediary evolutions for the existing species of otter Pokemon. According to Fantastico's interpretation of otter Pokemon ancestry, it all began with a creature by the name of Ottermatic. This ancient proto-otter would eventually form a symbiosis with fan-favorite Cloyster, which would then lead to all manner of other creatures, including the Pokemon franchise's contemporary otters, such as Samurott and Floatzel.
As was the case with Fantastico's dog Pokemon ancestry chart, here too they bring together species that might not seem immediately related in any way, shape, or form. This is where the Slowpoke evolutionary line comes in, which Fantastico imagines might've split off from the baseline otter evolution in ancient times. Since Fantastico needs to figure out how to best combine various different Pokemon otters in a way that makes sense, it goes without saying that much of the evolutionary tree is, in fact, custom in design, and does not appear in any of the official Pokemon IP materials.
Custom Pokemon designs are hardly a new concept, with the unofficial term of endearment for these community creations being that they are "Fakemon," such as Tniseraaso60s Ghost-type Heracross concept. What sets apart Alex Fantastico's work from most other community creations is how well-thought-out and organized they are, and this is bound to continue as they come up with more evolutionary lineups, too.
With Paradox Pokemon featured in Pokemon Scarlet & Violet, it's clear that Gamefreak has no problems adding new subspecies of existing creatures to the mix. Still, it's highly unlikely that the studio would be capable of balancing all the unique and curious creatures that Fantastico's come up with for his work, even if the developers wanted to adapt a similar concept sometime in the future.