Find Steed allows you to summon a spirit that assumes the form of a steed, creating a long-lasting bond with it. You can't have more than one steed bonded by the spell. It also says:
You can also dismiss your steed at any time as an action, causing it to disappear. In either case, casting this spell again summons the same steed, restored to its hit point maximum.
As an action, you can release the steed from its bond at any time, causing it to disappear.
Assuming your steed became true polymorphed into a small stone statue, and the DM rules that even if you dismiss and resummon it by re-casting the spell, it remains in that form.
Can you release that stone-statue beast from your bond, and bond with another spirit by casting Find Steed again?