Blame this.
Aqualions
If you are asking yourself “Why would anybody mutate a lion by giving it gills and making it amphibious?,” congratulations! You’re not a Mad Scientist. Mad Scientists never ask themselves questions like that. They’re often not self-aware enough to ask themselves “Why wouldn’t people mutate lions?” Mutagens are there to be used. Of course.
Of course.
In this particular case, Aqualions were the brainchild of Captain Sir Humphrey Walsingham-Reynolds, RNR, RCS, KCB, etc. etc. etc. According to his scattered, decaying, and somewhat gnawed notes the idea was to create a mascot for British ships of the line: Sir Humphrey was apparently quite upset that ‘sea lions’ did not even remotely live up to his mental images of them as a boy, and felt that the Empire needed something with a bit more puissance behind it. So he went and created lions that could breathe water as well as air (they needed the air in order to roar, of course). And could swim effectively underwater. And then he increased them in size to about one ton each and made them more aggressive, because why not?