A personal headworld wiki
This is a VERY loose guide, there aren't many hard no's. Any of the rules mentioned here can be bent a little bit to suit a certain idea/aesthetic. The only thing is they should still look like they are a crystali, and not some random other creature.
Crystali are your typical anthro/digitrade creature. They are covered in varying lengths of fur. They can have hair, or a mane, tufts of longer hair here and there, etc. Most things are fine in that department! They have pawpads on their feet, but not their hands. The hands just have really short fur most of the time.
The shape of their muzzle can vary. They can be dainty, or they can be thicc, just make sure it suits the species and isn't too far in either direction.
They are born with long rabbit-like ears, however a body-mod that is very common in Crystali culture is ear docking, so they can also have short ears. Ear docking originated from their military, where it is mandatory to dock in order to not have them be a hazard during fights. This later spread to laborious jobs and just generally became kind of fashionable. When it's done properly, the ear just looks shorter, but some poorer families will do it themselves and it tends to look rough then. It's actually pretty common for them to get torn as well. Generally, wealthy Crystali who don't work hard jobs will have dainty, fancy ears.
Their Claws are made of the same type of crystal as their head and tail ones, just claw shaped.
They do not have tails, instead their spine literally ends in a cluster of crystals. Every Crystali's tail crystal emits some sort of glowing/mist-like trail of magic. It can vary in brightness/size depending on the owner's health and magical ability. Some additional (small) crystal growths can occur along their spine and on their skull, but they don't appear anywhere else.
Their head crystals grow out of their skulls like horns do. they are born with smaller, nubbier crystals and they grow and mature with age.
Speaking of aging, Crystali have a pretty large variation in life expectancy. Weaker/average Crystali tend to live to be around 80-100, similarly to humans. Powerful ones can live much longer. Usually not longer than 300, but it wouldn't be impossible for someone to live way longer than that given the right circumstances. I would say about 75% of them are average, 23% of them can go up to 300 and the remaining 2% can go from 300-1000.
Most Crystali have more neutral fur colors. A few have brighter colors, but it's pretty unusual and they tend to stand out a lot.
Their crystals, claws and eyes are always the same hue. Most of the time they are the same color, but if not they are very close to it.
The colorful markings in their fur are also always the same color as their crystals. They're visual representations of their abilities. They can be born with them, grow up and start developing them, or they can intensify/grow with training. They are usually rune-like but they could be pretty much anything. They just should not cover more than like 20% of their body at most.
Wings are extremely extremely rare! If a Crystali has an air element I can see them conjuring wings to fly or glide, but true wings are very very rare.
Twilax pretty much breaks all of these rules because she was unfortunately one of the first ever born with no magic whatsoever. As such she has weird mutations where her teeth are crystal, she has no horns, she got a regular tail instead of a crystal growth and she has colorless fur/no magical rune markings.
Crystali with no magic can't usually use magic, but Twilax is able to channel magic with crystals that don't belong to her. She still has enough of a connection to Oerba to cycle the magic through herself and control it, but she needs someone else's magic as a base.
She was already regarded as some sort of evil creature that was rejected by god and not granted any magic, but she was completely outcast when she started attacking others for their crystals to use their magic. Although she was not born with magic, it seems she has the presence of some sort of plasma element inside her. Whenever she touches a new crystal it will change color to teal to match her element. She's basically in a perpetual state of hunting for crystals as as a resource.