To dream of a fire hydrant represents an emergency option that you are aware of, but never use. A fail-safe or backup plan to deal with a problem if it gets out of hand.
Negatively, a fire hydrant may be a sign that you are always prepared to overreact or “throw the baby out with the bath water” if things get bad.
Example: A man dreamed of seeing a yellow fire hydrant. In waking life he was always prepared to kill himself if his health problems got any worse, but they never did.