Answer:
A leathery snake, with a stinging bite. I'll stay coiled up, unless I must fight.
It doesn't live within a house, nor does it live without. Most will use it when they come in, and again when they go out.
It comes only before, it comes only after. Rises only in darkness, but rises only in light. It is always the same, but is yet always different.
He has married many women but has never married.
When does Christmas come before Thanksgiving?
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.