There are many ways to create new creatures in D&D 5e, one of which is animate objects which gives the player full control of the creatures when he/she uses a bonus action:
As a bonus action, you can mentally command any creature you made with this spell... You decide what action the creature will take and where it will move during its next turn, or you can issue a general command, such as to guard a particular chamber or corridor.
There seems to be no such language for Find Steed but does say:
Your steed serves you as a mount, both in combat and out, and you have an instinctive bond with it that allows you to fight as a seamless unit.
Does this mean that player does have full control of the steed or is it still an NPC and therefore under control of the DM?