Design

When it comes to bolidae, it's easier to list what they don’t have, rather than what they can have, as there are very few limits to their biology and appearance.
To give an idea of the most basic appearance of a bolidae:

This base is not the required elements of a bolidae, but is the best starting point to go off of when making a bolidae!

When it comes to changing these base design elements, bolidae can have:

More or less existing limbs or organs.
Any colors or variety of fur.
Varying height, size, weight, or proportions.
Any lengths or types of body fur and hair.
Fur patterns (with some color limits).
Any horn shapes, spines or other exoskeleton parts.
Can be anthropomorphic or quadrupedal.
Hooves or paws, even with webbing.
Scars, visible innards such as skeleton, or completely altered inner organs.
Etc...

So to better narrow it down, what can’t be changed? Here's what should not be added or altered:

If the bolidae has a mouth, it must retain both the nose and muzzle shown in the base.
Their ears can vary in size a tad, and even be floppy or crooked, however they cannot be any other shape than the one shown if they have ears.
They cannot be given entirely new limbs or outer organs, this means while you can add or subtract their arms, legs, eyes, or lungs for some reason, etc. you cannot give them new parts such as independent wings, genitalia, breasts, or a mermaid or serpent tail.
When it comes to fur patterns, their fur is mostly monochromatic, with minor hue shifts. Meaning, they may be blue with navy stripes, but not blue with red or green stripes.
The defining feature of bolidae are their tails. Although they’re not required to have a tail, ones that do have a translucent or opaque smooth, gel or goop-like, appearance, with their tail-bones visible inside or poking out the top. It can come in a variety of shapes, and never has any fur on it.
They never have talons/bird’s feet, a beak, or most other non-mammal features. Bolidae are mammals, mostly built for land, even if they are aquatic or aerial-themed. For example, they may have patches of feathers, or scales with gills, but will not be completely scaled or feathered with an actual fishtail or wings.
Their horns are synonymous with the rest of their fur, and have no bands.

So, while that may seem like a lot to memorize, don't fret too much about following those rules perfectly! There's no penalty or shame in bending the rules. In the end, they’re just suggestions, since at some point a character may stray so much that there's not much of a point in calling them a bolidae anymore.

In-depth look

Next, i'd like to share some things that I include in many of my bolidae designs and further explain their features! Some of these traits are not required of them, but are unique to their species.
Click these to jump to certain points:
[ False Teeth/Forked Tongues---Spines/Exoskeletons---Horns---Eyes---Ears---Tails ]

1. False teeth and forked tongues

Many bolidae have false teeth on top of their real ones. Their teeth are usually carnivorous, and false teeth are very similar to their horns but simply around their mouth. They can be on top, or more like tusks on the bottom. There is no limit to how many false teeth they can have or how big or small either. Alongside this, forked tongues are rather common too!

2. Spines/Exoskeletons

This is technically a mutation, but is a very common one I like to include in designs!
Oftentimes this simply entails their spine poking out their back, or ribs being on the outside, but this can include ANY of their bones, even their skull.

3. Horns

Most bolidae have horns! Their horns can be placed anywhere on their skull, and be any shape. This means they can resemble antlers as well. These horns are always covered in a velvet that blends into their fur seamlessly, and their horns have no banding. They do not shed the velvet nor the horns. Horns might be broken or damaged though, unless the bolidae has some special regenerative ability. Since the horns do not grow over seasons, broken horns will stay broken. Maybe try gluing the horn back together I guess.

4. Eyes

Bolidae can have just about any design for their eyes! Not just mutations either, I typically get creative with their pupils to make their design more unique.

5. Ears

When it comes to their ears they may vary in size and fluffiness, some can even be floppy. But one thing should always stay consistant: their shape. Their ears point up and outwards, with a large teardrop-like shape. Most comparable animal's ears would be the fennec fox's if you need an irl reference.

6. Tails

One of the most unique features of bolidae are their tails! They never have regular tails like any other animal. The only exeption is if they don't have a tail at all.
Bolidae's tails are a gooey-like limb, with no skin or fur on it. It typically has some amount of transperancy, although never fully invisible. They can come in just about any shape! Whether or not stuff like veins or blood can be seen in the tail or if it bleeds at all is up to each individual creator. The tail bones will always be visible though, some may be fully contained in the tail, while some may poke out the top like their spines.