General Guide

Head feelers are one of the most important parts of a pacapillar.
All pacapillars have at least one head feeler; they help with gathering information about their surroundings: they help Pacapillars smell, feel the air, detect heat and vibrations. They can come in an infinity of shapes and sizes!
The examples in this guide are by no means an exhaustive list of valid shapes, traits are open-ended, so feel free to experiment with other shapes and variations!

Common head feelers are 1-2 pairs of head feelers, typically small and simple-shaped. Their length does not exceed the size of the pacapillar's head.

Rare head feelers allow for more complex shapes, such as animal ears or wing-like shapes. Rare feelers can be longer than a pacapillar's head, but may not extend further than the height of their whole body. Having 3 or 4 pairs of head feelers is also considered Rare.

Super Rare head feelers may extend beyond a pacapillar's body height. Having more than 4 pairs of head feelers is also considered Super Rare.

Similar to head feelers, cheek nubs help the paca gather more information about their surroundings. These mostly supplement the function of head feelers, but they are located in the cheek area.
Since their function is not crucial, it's common to see pacas with no cheek nubs at all! They also come in an infinity of shapes and sizes. 

Common nubs are 1-2 pairs of nubs that are no longer than half a paca's head height, not counting the size of the head feelers. These are simple shapes, including but not limited to; teardrops, cones, ovals, leaf, heart. Having no cheek nubs is also common. 

Rare nubs cannot be longer than a paca's head height, not counting the size of the head feelers. They can be in any shape. A rare paca can have up to 4 pairs of cheek nubs.

Super Rare nubs are longer than a paca's head height. They can be in any shape. Having more than 4 pairs of cheek nubs is also considered super rare.

Tail feelers are extremely varying in shapes! They can have any shape. The function of tail feelers varies from paca to paca, since they are so diverse, some help with better balance, others are just for show. Some pacapillars don't even have any tail feelers! Their rarity is based solely on length, however, the texture of the feelers must still match the texture of the paca.

Common feelers can reach up to half the height of the pacapillar's body (measured from head to belly, not including the legs).

Rare feelers can reach up to the height of the pacapillar's body (measured from head to belly, not including the legs).

Super Rare feeler size can reach up to 1.5 times the height of the pacapillar's body (measured from head to belly, not including the legs).

Anything longer than that is considered Ultra Tail, which is a Mutation+ trait.

In general, shapes for these three parts of pacapillars are very diverse. When designing your pacapillar, remember that the sky is the limit! The examples in these pictures are just that: examples. They are meant to aid, not to limit, in the understanding of traits.

Textures are, basically, what a pacapillar appears to be made of. Such as slime, piñata paper, flower petals, feathers, etc...
There are many different textures, and each texture can have variations (for example: plant texture includes flower and plant petals, but it also includes things like mushroom or cactus texture). A paca with a smooth, "normal" texture is classified as matte.

You can read the description for each texture in the encyclopedia.

Common: Matte, Fur, Spine, Sponge, Chipped.
Rare: Slime, Patch, Feather, Scales, Shell, Mist.
Super Rare: Crystallized, Plant/Petal, Piñata.

Extra traits are traits that are not included in a Pacapillar by default, but they can be added to make for a fancier Pacapillar! 

Common: Pawpads, Extrusions (Hair)
Rare: Extrusions, Collar/Mane/Hood, Mismatching, Fangs, Hollow, Horns, Bioluminescence
Super Rare: Transparency,  Aura/Halo, Ribbons, Mirror, Wing Shades

Mutations are special traits that are normally not present in Pacapillars. Each mutation has their own rules, which are described in detail in their encyclopedia page.
There are two categories of mutations: Mutation and Mutation+ (read as Mutation Plus). Mutation+ traits are different, more rare, and often (but not always) are variations of normal Mutation traits.

These traits are unlocked using an Olera Berry for Mutation, and a Ripe Olera for Mutation+. Each berry gives access to one trait, so if you want to add two mutations, for example, you would need two Olera berries. There are also seasonal items for Mutation+ traits, which give a specific trait depending on the item; you'll be able to learn more about berries and traits in the MYO Guide!

Mutation traits are Pincers, Multi-Texture, Shedding, Single Tail, Slug Body, Split, Chameleon, Paws, Spider Eyes, No Eyes, Body Eyes, Cyclops, and Animation , you can read their specifications in this page.

Mutation+ traits are Ultra Tail, Multi-Texture (Plus), Octo Body, Object Body, Object Head, Overlay, Split (Plus), Chameleon (Ultra), Fins, and Animation+ (Segmentation), you can read their specifications in this page.

Subspecies are different Pacapillar types that have different features and sizes, which do not conform to what a Standard pacapillar would be.
There are two types of subspecies: Natural subspecies, and Unnatural subspecies. A pacapillar can be born as a natural subspecies, but it's impossible for one to be born as an unnatural subspecies. Unnatural subspecies are a result of a transformation caused by a pacapillar's life events; it is something that occurs in extremely rare circumstances. 
In exceptional cases, a pacapillar may be a cross of two different subspecies, this phenomenon is so rare that is yet to be well-documented.

Natural subspecies: Gilled, Poison, Hard-Shelled, Soft-Shelled.

Unnatural subspecies: Winged, Monster.

Subspecies guides are still under construction, we appreciate your patience!