I think that none of the original forms of the homunculi are given an explanation.
- Lust has the body of women that can change her fingers to needle
- Wrath is a mortal homunculus that has a human body.
- Sloth has that big giant kind of body.
- Envy has that green monster thing as the original form that he
disguises to a normal human body.
Only Gluttony can be given a reasonable explanation for his original body. He is the failed gate of truth and hence has a false gate in his stomach.
You can argue Envy has that green monster body since he is a compilation of many human souls but that is true for all the homunculi, so we can't take that as an explanation.
Also, I went through the WIKI and couldn't find anything on this.
I also think the fetus body remained in the end because Pride was removed (killed) from the body and only that fetus remained symbolically telling that the monster has been removed and only a pure form of the being (baby) remains(even though the baby is a homunculus).