The first step when thinking of how to draw any kind of mutant is to determine their origin story, their life before the mutation event took place. Whether it's a heavy dose of radiation, genetic splicing or a bit of magic, the 'how' behind the mutation will inform your reference research.
Don't be afraid to get specific when gathering your reference material. Mixing in elements from another particular species or exaggerating known and existing mutations can help add flavour and believability to your design.
The level of mutation you create, from as subtle as a slight shift in bone structure to severe body form changes, will entirely depend on the needs of the story you craft. For this article, I'm going to illustrate a woman who's been bitten by a cursed spiny orb weaver, a type of spider that's striking in both form and colour.
Working in Photoshop, I began with a sketch of a human in the pose I want. Taking this approach can help you keep mindful of the flow of the forms, creating a natural mix between the human face and the mutated elements.
Maintaining enough of the human form in the face can create an even more disconcerting mutant, because it can still evoke empathy in the viewer.
01. Concept sketches
On the left, I start with a human form in the scene to give myself a base with real anatomy to alter for the mutation. On the right is the spider mutant sketch I create with a mix of lassoing elements to copy and free transform them, before painting over them to build out new forms.
02. Human touches
During the render phase, I add an earring to my figure as an extra touch to show humanity. To make a clean hollow circular shape you can use the Elliptical Marquee tool in Quick Mask mode (press Q). If you mask the area you want selected, a quick invert selection will do the trick.
03. Mutant textures
Incorporating the different ways that a creature's skin pits or shines in the light can help create contrast to the formerly human form. To help add in some of these textures I use a variety of palette knife-style custom brushes made by Kyle Webster.