Shapechange is a 9th level spell that allows you to assume the form of a different creature, transforming into an "average example of that creature".
There is at least one creature with random chances of extra things, the Frost Giant Everlasting One :
Extra Heads. The giant has a 25 percent chance of having more than one head. If it has more than one, it has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, or knocked unconscious.
If, for instance, a Wizard or Druid player character would be to Shapechange into a Frost Giant Everlasting One, would they always get a single head (since, on average, 75 percent beats 25), or would they roll for it ? What happens when you Shapechange into a creature with random chances of extra things ?