This is actually pretty easy to mod in as it stands. Just create a custom entity file that has a whole bunch of different creatures linked inside of it, and you have a lot of weird options to play as in adventure mode.
However, as others have mentioned, there are a lot of problems with playing any creature that wasn't set up with the idea of being played. Beyond just being attacked by your own kind in the kobold situation, most creatures lack the tags necessary to go on even a very basic adventure.
With no [GRASP], you can't carry anything--including food, so the length of your adventure is limited to however long it takes your creature to starve.
With no [CAN_LEARN], your stats never go up, so you stay effectively a "level 1" of whatever creature you are until death. With no [CAN_SPEAK], you cannot interact with any other person in any town, and so cannot get followers, reputation, or anything.
Even more major than that, with no [CANOPENDOORS] tag you can have your adventure end as soon as it begins if you happen to spawn inside a building. You cannot open the door, and so you literally cannot advance.
These can all be overcome if you really want to play as some particular creature--you can just go into the RAW file for whatever creature you want, and mod it until it can do what you need. I regularly play as dragons, and so just mod the dragon's raw file to have [CANOPENDOORS] and use a specialized body that has [GRASP] parts so I can carry food. But if you just select a creature at random, most likely you will not have the right tags.