The only rule is that you have to live like the one that you choose does. A few examples?
-If you're a Rayquaza, you have to endlessly protect the Ozone layer. No slacking off.
-If you're a Regi, you have to handle being locked away.
-If you're a Magmar you have to be in a hot volcanic location.
-If you're a Mimikyu, no one loves you.
-If you're an Eevee, you may not be able to pick the Eeveelution you get down the road in your life, it may be picked for you (even if you're not caught)
-If you're a Cubone, your mother died. Sucks to be you.