1. What is it? (and why is it)
An elastic hound, otherwise known as a Gargarus Hound, is a dog like creature with the ability to extend its limbs and neck up to a yard or more. How it accomplishes this is a mystery, but it has already been ruled out that it hides it's limbs in some way to give the illusion of growth. Whenever they are killed Gargarus hounds limbs recede back to their original length. It's origins are also somewhat of a mystery, but there are many competing theories. One definite fact is that they appeared roundabout 150 years ago in the southern reaches of Vorhil, and were discovered by a traveling bard who witnessed a group of them attack and overpower a caravan. Some say they are the creation by a dark wizard (I'll flesh out the backstory here), the demonic spawn of (name of dark god/goddess here), and some say that they are simply nocturnal creatures living in caves and tunnels, who adjusted to the light.
2. What does it look like? (physical description)
Gargarus hounds are often covered in a thin brown fur, some with spots and others without. Alpha males tend to take on a golden coat for whatever reason, but both types are very lean and thin. Their jaws are large and rounded, with big under bites protruded with sharp teeth. The most obvious trait of a Gargarus hound however, are its eyes, which shine either a very bright green or blue.
3. How do you, or how would you use them in any given rpg?
Initially to catch the player off guard and ambush them. After that just as special enemy types. Perhaps if one could find the right persuasion they could adopt one of the hounds as a pet or loyal travel companion. Maybe have a small quest where an NPC is selling puppies, but isn't selling real puppies, but clones or something equally nasty that ends up causing havoc.
4. Any and all cool details...what does it eat? where does it live? does it have any weaknesses?
Eats meat, lives in woods or mountain areas, common to find in a small group of up to 4 hounds. Very intelligent, almost with a human capacity. Mostly comes out at night. Mostly. (perhaps weak against light based attacks?)
5. Does it have any special attacks?
1. Extending its limbs to wrap around their victims and suffocate them with their tongues.
2. Extending it's neck to bite from a distance.
3. Can trip players with limbs/tongues
4. Knock items/weapons out of players hands
5. Has the ability to reach around shield defenses, or anything similiar
That's all I've got for now. I'm going to flesh all this out later and post whatever it becomes. thank you for the advice so far! It's very helpful.