(The above designs are NOT mine, but a very good friend on Deviantart allowed me to borrow them for reference on appearance. Find her lovely designs here:
http://tailmouth.deviantart.com/gallery/ )
So. Firstly, their appearance. There really is not limit on the color or pattern a tail-mouth can be. And yes a Tailmouth can have up to two tails. But this causes much strain on the host. Why? I will explain.
Tailmouths actually are scarce in terms of their population. The largest pockets of them can be found in the Northern half of Odaigra; within the vast crystal caverns. The few that live in the Southern half have less fur on them due to it's usually warm climate. Combined with the complex relationship between them and their mouth tails (or Amosiruo) makes for not very large pockets or settlements.
Tailmouths (both male and female) are usually very slender with their strongest muscles being in their legs, back and upper torso. This is usually accounting for the shape of their mouth tail. Shorter, puffier mouthtails are accompanied by either short or stout bodies. Though this is not a set rule either. Any kind of body type can match with any mouthtail; which makes for a very interesting and visually diverse species. Not all kinds of mouthtails are visible in the above picture so I will list them:
-Traditional (the one you can see in most of the reference pictures)
-Sharkstyle: has a stronger overbite but are usually more aggressive
-Whipstyle: has a detachable lower jaw (like a snake) which can swallow larger prey
-Ravenous: Most deadly and violent, oozes acidic saliva that can melt bone and muscle. Few tailmouths born with these tails live past childhood
-Cutterstyle: shaped exactly like the traditional though slightly shorter, teeth vary in shape and texture (serrated or otherwise) making for excellent slicing strength
-Viper(or Striker): Much thinner than the other styles, acts and behaves very much like a snake. A few known tailmouths with striker tails have become assassins.
-Cursed: Nearly half the flesh and fur are burned off these mouthtails at birth. No one is sure why. Said to be half demon, these tailmouths often live in solitude for fear of rejection and hate.
Tailmouths, though primitive in dress and technology, are not in any way ignorant or stupid. Though it takes them quite long to learn physical language, they are capable of using telepathy and often speak with each other using such. Thus you will rarely hear children speaking to one another should you come across younger tailmouths. the conversation is probably happening in their head. Elder tailmouths however use spoken language; often speaking in a combination of Draconian and Elvish (seeing as how dragon like species and nymphs (a kind of spirit elf) live in their realm.) Tailmouths who venture outside Odaigra are usually the ones to learn English, and bring it back to teach others.
Most Tailmouths knowledge however is very much limited to what goes on within Odaigra. Only a few small groups or pairs know of things that lay beyond. This usually is because of many Tailmouths lack of curiosity or care of goings on past their home. It is a widely accepted train of thought for Tailmouths to give little attention to anything outside.
Curiosity is rare amongst them; and while not looked down upon, it is usually not celebrated. Especially in children.
NOW onto the relationship between a Tailmouth and their Mouthtail counterparts. It is very complex but I'll try to be as brief as possible.
Tailmouths usually grow their mouthtails around the age of 100 (1), and are usually fully grown by 1,000 (10). Tailmouths live with their parents until they are around the age of 16,000 (16) after which they are allowed to leave and live by themselves. By this point most generally know how to find food and speak some semblance of spoken language.
Mouthtails have minds of their own. Separate consciousnesses that works sort of side by side with their host Tailmouth. IT is a less intelligent mind, relying more on instinct than complex thought, but they can have varying personalities. Ranging from docile to violent, energetic to lazy and so on and so forth. USUALLY (not always) the personalities between them are very different, causing some conflict between the two. And if a middle ground is not reached soon the two can engage in a violent strife, sometimes (60% of the time) this can result in death of either both or one of them. Usually only one side dies, whether that be the mouthtail or the Tailmouth. Whichever side lives it discards the dead weight and either leaves it behind or consumes it for it's life energy.
Tailmouths can become violent or hostile right out of birth, should this happen they usually attempt to devour their parents. Since Tailmouths don't grow mouthtails until later, the parents sometimes end up having to kill their offspring for fear of endangerment to themselves and others. They may or may not attempt to have more children afterwards.
Some are born with complimentary personalities and it takes little time for harmony to be reached. Tailmouths also share a single digestive system with their mouthtail, regardless of which side eats food, both sides will benefit.
(I'll add more later if I feel as though I should!)