They're carryovers from the old BIOWEAPON Eclipse worldbuilding, which was called Tellurian Chronicles and took place in space among many extrasolar planets similar to Star Fox. When I moved everything to being on Earth and Mars, I didn't want to ditch all of the worldbuilding, so I just fit it into Earth. The Argonian Republic became the American Republic and the Gatanese Empire was basically split into the Dragon Empire and the Chimera Regime.

You can read old Tellurian Chronicles and BIOWEAPON comics on Tapas.

It's beyond physical strength. Therians have better senses and physical abilities based on their species, like increased speed or a prehensile tail. They also have DNA beyond just their specific animal. They have tardigrade DNA to make them resistant to radiation, which helps them survive on Mars. They have axolotl DNA to increase healing ability, though most don't regenerate like axolotls. Only special further experiments resulted in individuals with regeneration and, an extreme result, shapeshifting. These would be Rubi, Azura, and Jaron.