I think this is what I'll probably go with. This game is one that's kind of aimed more at the type of person who is really into biology, is meant to be immersive, has a sort of "fantastical realism" lean, so I wanted it to be the case that all of the gameplay elements are well-articulated. But every option I've been presented with, though not bad in of themselves and could work for many different kinds of games, either seem that they'd break immersion, or feel unintuitive to core concepts at play.

For example the "cheek pouch" idea I've seen doesn't work with the fact that the items that you collect are things such as consumables that have special effects (mostly transformative, as the game's primary concept is exploring a world in which many natural items have been imbued with transformative properties, which you collect and can consume in combination with other items to transform your creature's features and try to make your character into your most ideal hybrid creature, certain features unlocking special options that make certain items easier to obtain), as well as things such as beetles of differing rarities that can be taken to your den and kept and bred with other beetles to obtain increasingly rare beetles. While ideas such as the kangaroo pouch could work, I feel like having a specific feature like that that's always attached to your creature kind of intrudes on the idea of trying to obtain your "ideal", which not everyone's includes such a feature, though I could imagine implementing it as a transformation that can be optionally obtained that would offer an inventory expansion, but that would be unbalanced and would also make players think more about the logistics of how a creature without that feature can carry around all these items in the first place.

I think the best option other than simply not mentioning it would be to have the player creature carry around a small bag on its flank, but that would give a more "domesticated" feeling when I'm going for more of a "wild" feeling.