Find Steed (p240 PHB) States:
When the steed drops to 0 hit points, it disappears, leaving behind no physical form. 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.
If I were to cast True Polymorh (p283 PHB) on the steed and wait the full duration to make it permanent, would the steed return when summoned as the original steed or as the new creature it was polymorphed into?