The Mutant Crawl Classics core rules have tables for determining mutant appearance, manimal subtype, and plantient subtype. Each table takes a similar form, roll 1d30 to pick a category of mutation (skin color, skin texture, eyes, etc for mutant appearance; primate, canine, feline, etc for manimal subtype; deciduous, conifer, fruit-bearing, etc for plantient subtype) then roll again to choose a specific mutation from within that category.
There isn't necessarily a problem with any of these tables, but I do worry that there may be slightly too few specific options within each category, and that the categories for manimals and plantients leave some obvious choices out. (I also wonder about some of the mutant appearance charts. Is there a meaningful difference between hands that "are comprised of tentacles" and hands that are "absent, replaced with tentacle fingers?" What if someone's sole mutation is to have 2 arms or 2 legs, which is a serious possibility on the "Body" category subtable?)
Partly, this concern is practical. Tables with too few choices risks producing too many repeats, and because each MCC game begins with 4 characters per player, this risk is greater than in other games. (Reading the Random Esoteric Creature Generator and then reading Island of the Unknown shows what this risk looks like.) It doesn't matter if the mutants at one table look like the mutants at another, but it does matter if some of the supposedly unique mutants at one table look like others at the same table. (If mutants are supposed to belong to distinctive types, rather than being individuals, then this isn't a problem at all, but I think that the mutants are supposed to each be unique.)
The other part of my concern is creative. I don't claim to have imagined every type of mutation, but it seems there are some obvious possibilities left out. Many of the mutants depicted in the art of the book, for example, cannot be created following the tables, which seems like another missed opportunity.
In the future, I'll write my own tables, but for now, here are three alternative tables based on the mutations in the Crawling Under a Broken Moon zine and the Umerican Survival Guide. These tables provide categories of mutation without offering specific types within each category. One way to use these tables would be for the player and judge to decide together what the mutation looks like. Another way would be to follow up by rolling on the CUaBM or USG mutation tables, which provide chances for a number of possible abilities related to each category. One caveat here would be that these tables were originally written for a world where mutations are less common than in MCC.
The first table provides alternative mutant appearance results. The second provides alternative manimal subtypes. The third provides alternative plantient subtypes.
Mutant Appearance (roll 1d24)
1-2 Extravore (digestive abnormality)
3-6 Extra limbs
7-10 Cranial abnormality
11-14 Dermal abnormality
15-18 Ocular abnormality
19-20 Aggregate features (roll 1d3, 1 granite, 2 iron, 3 crystalline)
21-22 Weaponized features (roll 1d6, 1 claws, 2 spines, 3 fangs, 4 horns, 5 club fist, 6 spikes)
23 Roll 1d20 once on this table. The mutant has two mutations of this type.
24 Roll 1d20 twice on this table.
Manimal Subtype (roll 1d20)
1-2 Avian (bird-like)
3-4 Bovid (cow-like)
5-6 Canine (dog-like)
7-8 Crustacean (crab-like)
9-10 Feline (cat-like)
11-12 Piscine (fish-like)
13-14 Ambphibian (frog-like)
15-16 Reptilian (lizard-like)
17-18 Testudine (turtle-like)
19 Roll 1d16 twice on this table.
20 Roll 1d16 once on this table and 1d20 once on the Mutant Appearance table.
Plantient Subtype (roll 1d12)
11 Roll 1d10 twice on this table.
12 Roll 1d10 once on this table and 1d20 once on the Mutant Appearance table.