I always got the impression that the grey seal was added first because there was a relative dearth of European animals at the time. Plus the California sea lion was an obvious choice for a North America pack.
There surely must have been some planing so id assume they didnt do sea lion in aquatic because it was intended to come in NA laterAll the aquatic pack choices were not the most obvious ones. The African or Humboldt penguin, California sea lion, Asian small clawed otter and spectacled caiman or American alligator would have made more sense, unless Frontier was saving them on purpose for later packs.
The penguin and caiman were pretty logical choices in my opinion tho. If you hear penguin you probably think of antarctica, ice and snow so doing king penguin as the first penguin makes sense to me, not to mention that they are probably the most iconic one. And if they did one of the banded penguins instead there really wouldnt have been an opportunity to do an antarctic one later. As for the caiman, doing a crocodilian was a pretty boring but probably needed choice seeing how they did a whole new mechanic, and 3 wholey new animals, so something simple was probably needed.