Black Maria ate the Kumo Kumo no Mi, Model: Rosamygale Grauvogeli, an Ancient Zoan-type Devil Fruit that allows her to transform into a hybrid and full version of a Rosamygale grauvogeli, an ancient species of spider.
Back in Chapter 998 we discovered that unlike the Tobiroppo who were portrayed in their full zoan form, Black Maria was in a strange transformation that mixed full zoan and full human.
In SBS Volume 100, Oda not only explained the reasoning behind Black Maria’s unusual half-beast look, but also provided a rough illustration of how it would actually look like.
Question: Black Maria’s ability is kinda like the SMILEs, no?
Oda: I know what you mean! I know that the hybrid form shouldn’t have two faces and her arms should be spider-like. However there is no clear cut rule for the zoan transformations, see Chopper. Just like he manipulated his transformations, it is possible for others as well! Black Maria also feels that becoming a full spider woman isn’t beautiful at all, so she must have used some drugs to change her Hybrid form. Here’s the natural one by the way.