Valenth

A side-view of a "standard" leupak.
Mouseover for internals.
I. Facet. II. Tongueclaw in rest position. III. Stomach. IV. Venom gland.V. Egg sac.

Leupaks are anatomical anomalies. As beings made entirely of dreamstuff, they are capable of violating the laws of physics, some more than others.
They come in countless forms, infinite varieties-- and yet, all of them share the features listed here. They are what make leupaks leupaks-- whatever they may be.

First and foremost, they are always reptilian-- I have seen leupaks that mimic certain extant mammals, leupaks with hairy manes or decorative feathers, leupaks without scales, and other such things. Yet, they are always cold-blooded, lacking mammaries or mammalian tendencies.

Many leupaks are quadrupedal simply for the sake of efficiently bearing their weight, though all can stand on their hind legs for a short while by balancing with their tails. Bipedal leupaks tend to be bottom-heavy with short, thick hind legs.

Quadripedal (left) and bipedal (right) leupaks.

All leupaks born of the Crystal possess an organ known as the facet (I.). The most vital part of a leupak, it contains its dream-essence, the concept that brought it to life and sustains it much as the heart and brain sustain a natural beast. No leupak can survive the removal of its facet.

The tongueclaw (II.), found in all leupaks and pseudoleupaks (see venom), is a hollow, muscular fifth limb with several special functions-- venom delivery, ovipositor, and siphon, as well as manipulating objects and as a sensory organ. The venom glands connect to the tongueclaw at its base, just before it splits into two sections; one leading into the stomach (III.), the other becoming the egg sac (V.).
On average, a leupak's tongueclaw is about half its body length sans tail, and can safely extend half this length from its mouth.

The tongueclaw extended in display, digits spread.

When at rest, the tongueclaw's digits fold under and tuck beneath the main tongue to allow ease in speaking and eating. In use, the claws spread and extend, functioning much like a prehensile hand with an opposable thumb.

Like a chameleon's tongue, a leupak's tongueclaw can be shot rapidly from the mouth like a spear in order to punch into prey or hosts to siphon blood, place eggs or inject venom. When using the tongue as an ovipositor, a leupak's jaw unhinges to allow safe passage of eggs through and out of the throat. Venom is excreted from pores just within the tongue's hollow opening; it can be injected directly through the tongue, or let drip into the mouth and chewed into flesh.

Leupak hearts and lungs are surprisingly small for the beasts' size, kept well out of the way of their other organs and protected by the thoracic diaphragm. This is the most rigid muscle in a leupak's body, bending just enough to assist breathing-- I believe to protect the vital organs from being crushed by the stomach, egg sac, or any other consequence of the leupak's natural elasticity.

A leupak's muscles have precise control and are anchored to the skin, allowing the creatures to manipulate their bodies to a degree that defies modern science. I have personally witnessed many leupaks performing such tricks as crawling under beds and squeezing into spaces apparently far too small for their bodies, as well as quickly pressing otherwise dangerous bleeding wounds shut.
Some particuarly talented leupaks have taken this a step farther, using the technique to temporarily take a more humanoid form.
Generally, such tricks of self-compression are limited only by the size of a leupak's skull, spinal length, stomach contents, and vital organs such as the lungs and heart. Leupak rib-bones are multi-jointed with blunted ends to allow for maximum flexibility with minimum risk of damage.

The tongueclaw is, by far, the strongest muscle in a leupak's body. In leupaks of average size, it is capable of crushing force similar to that seen in large constrictors.

A leupak at rest.
Note the venom dripping from the jaws.

Leupak flesh-- from the outer skin and scales to the muscles and most of the internal organs-- is tough and extremely rubbery.
Aside from granting excellent protection against most creatures who might attack one, it allows them incredible flexibility and elasticity despite their girth. A special oil they secrete from their skins seems to provide this effect; it is useful in a variety of applications, including surgical, construction, and production of plastics.
For the leupaks themselves, it allows them to consume the vast quantities of blood necessary for egg and venom development in both genders, as well as safely carrying eggs in the final stages of development.

Leupak skin is useful for crafting watertight and mirajin-resistant laboratory wear, as well as zeppelin and airship balloons.