Changeling

A race of shapeshifting humanoids, Fork being the most relevant example.
As Fork has described to the party:

• They are inherently sexless, taking on whichever form they feel like, whenever they feel like it.
• That said, some changelings may have a preferred form, and therefor sex.
• While they can take the forms of other humanoid races, they do not gain access to the capabilities of them.  So they cannot fly as an Aarakocra, or see in the dark like a Tiefling.
• Should a changeling in a female form become pregnant, they are restricted to female forms until birth.

For a readily available description of the race, I reference the source material available at DnDBeyond.

Basic Information

Anatomy

As described by Fork;
Inherently sexless, and isn't aware what their true form would even look like at this point. Took their best guess, using what their father looked like as an adult as a reference.
Can change shape into a number of forms, but appears to be limited to medium sized humanoids. Their clothing does not change with them.
Expressed that for their kind, their form is akin to a facial expression. While an expression can be indicative of a mood, an expression can also be forced.