The Thing first appearance was in Charles Addams' 1954 book Homebodies. Well, that's not really true as that book is a collection of stuffs he did before for the New Yorker or TV Guide.
One of Charles Addams' cartoons represents his family's mansion with a sign "Beware of the Thing", which was adapted in the original series while the remake (The New Addams Family) has a sign "Beware of Everything".
There is also a second animated series, the one from 1992-1993, episode called "Beware of the Thing", now that I think of it.
They say Thing T. Thing firstly had an all human body and was cut for the series, but I can't verity as I can't put my hand on a Homebodies copy.
Some say that it (not Itt, the Thing) was Morticia's lover and they didn't foresee the fact that it'd still live after being separated from its now defunct owner.
We suppose it's a reference to the legend of Main of Glory, which would be the hand of a hangman or a murdering hand, could open any door and if one made a candle out of the fat, amongst other things, of the once owner of the hand and fixated it to the hand, it immobilize anybody to whom you presented it.
In the episode Thing is Missing of the original TV-Show as well as the remake, we can see a photography of Thing's parents, two hands.
Thing had a little romance with Lady Fingers, engaged and promised to wait for her.
Thing started in a box. He was then played by Ted Cassidy or some other member of the crew, mostly Jack Voglin, in the first series and as it will neverend (yeah, scared as well that it'll be a cleany cleantsy clean Tim Burton movie, like some of those he did recently) I won't say it ended being played by Christopher Hart or Steven Fox.