dj octavio found a axolotl and injected it with inkling genetics wich is why it has a similar build to an inkling and in game if you're an axolotling you can swim around for 30 seconds max and them you splat if you don't superjump out the water
Inktolings were fused by dj octavio in an attempt to make a terrifying octarian to hide among the inklings but he wanted them to have no suctions on the outside so he scrapped it and let them free
Yes, I know that, but how would they exist? The only way I can see them existing is a cloning machine that somehow fused some Inkling dna and Octoling dna
Maybe instead of swimming in ink they go into their shells and roll at enemies at high speed which spreads ink on the ground in the process (after typing that I realize that would be a better special ability than a default thing)