Every P.SE user has me.
We all have a little avatar which is art.
Scramble me, then not only P.SE users have me.
Not only P.SE users can have a rat as pet.
Then scramble me and prepend two letters, I am in the heart.
The aorta is connected to the heart if that counts as "in".
Then change one of my letters, I save people from impending doom.
Morta. Who else could better avert doom than a goddess of death? speaking from experience on that one ;)
Not so serious alternative:
Unless you mean a porta (potty), cause not having one when you need it can be a "doomy" experience as can be seen here: workplace.stackexchange.com/questions/87395