What happens when you Shapechange into a creature with random chances of extra things?

Question

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 ?

Answer 1

Average

(adj.)

1 : equaling an arithmetic mean

2a : being about midway between extremes

a man of average height

2b : not out of the ordinary : COMMON

on an average day

the average person

It’s not 1 because the average person has approximately 1 testicle and 1 ovary. Whereas any person picked at random is most likely to have 2 of one and none of the other.

It’s not 2a because midway between 1 and more than one head is one and a bit heads which doesn’t work.

That leaves it’s with 2b. It is not “out of the ordinary” for a Frost Giant Everlasting One to have one or more than one head so either is fine.

Whether the player chooses, the DM chooses or you roll each time is up to the DM.