Any idea what this means and how it originated? Animals teeth?
Old teeth/bas gums?
I think it does indeed come from the fact that as some animals age, their teeth look longer because the gums recede. Also, some animals’ tusks actually DO get longer as they age.
IIRC Horses teeth never stop growing.
So the longer they are the older they are.
Also the genesis for’ Don’t look a gift horse in the mouth."
But I could be wrong!
-Frankie
“Mother Mercy, can your loins bear fruit forever?/Is your fecundity a trammel or a treasure?”
-Bad Religion
The way I heard it, this phrase refers to the age of a horse. Supposedly as a horse gets older its gums recede further and further. An old horse would show very little gum line and would seem to have longer teeth, thus the phrase “long in the tooth”.
“It’s only common sense,
There are no accidents 'round here.”
Beaver’s teeth do not stop growing- if they have an overbite, thier lower teeth will continue to grow until it pierces thier skull, killing them. As a kid I had a Park Ranger show me a skull with this condition leaving an indelible impression on my mind.
So “long in the tooth” may apply to several animals and may not be because of receding gums.
I don’t have anything to add to the explanations of the other posters, except that I know their origin is correct because I read it somewhere!
Sits back with a statisfied smile on his face, happy to have provided a definite authority.
Actually, where I read it is here. http://members.aol.com/MorelandC/HaveOriginsData.htm#LongInTheTooth
The home page for the Phrase Origin site: http://members.aol.com/MorelandC/Phrases.htm
No, “don’t look a gift horse in the mouth” means when something good comes your way gratis, don’t start judging it’s worth.
As I understand it, one way the health of a horse can be assessed is by the condition of its teeth and gums. In other words, don’t question the value of a free-bee. Take it for whatever it’s a worth. Afterall, a horse is a horse, of course, of course! 
“They’re coming to take me away ha-ha, ho-ho, hee-hee, to the funny farm where life is beautiful all the time… :)” - Napoleon IV
Frankie right, too, Jinx; you cna tell the age of a horse from it’s teeth, so if you look a gift horse in the mouth you’re checking to see whether your freebie is a sprightly young thing or a nag about ready to keel over from old age. People even used to file horse’s teeth to make them look younger; not only do the teeth get longer with age, but they develop characteristic grooves.
My recollection is that it originated as a way of dating prehistoric cats - the longer the saber, or tooth, the older the cat – hence, the older cats were “long in the tooth.” I’ve never heard of its application to horse’s teeth, since I wouldn’t think their length is as obvious. However, if it works for you . . .
As a horse owner, breeder, trainer, I can attest that a horse’s teeth do indeed grow continuously throughout it’s life. They may occasionally need to be filed or “floated” to keep the angles correct so the horse can chew properly.
As the horse gets older this becomes harder to do. Instead of directly opposing each other, the horse teeth get longer, and angle outward. this makes it harder and harder for a horse to chew.
Eventually it may no longer be possible to float the teeth, and it may recquire a special easy to chew diet. After a time this may not be enough and the horse will starve to death.
Hence a horse that is exceptionally “long in the tooth” has one foot in the grave, and will recquire special treatment to maintain.
This is also the logic behind the phrase “don’t look a gift horse in the mouth.”