I agree with the accepted answer, which says that:
(1) Assuming a Medium caster, a size Large Steed could not be brought along as a willing creature due to the wording of the Dimension Door spell, but
(2) If the caster is mounted on the steed, the Find Steed ability to "make any spell you cast that targets only you also target your steed" means that even a larger-than-caster sized steed could be affected when the caster targeted only themselves.
Assuming that this is the correct interpretation, and that the steed is not the willing creature brought along but rather an independent target of the spell, do all of the features of the spell also affect it independently?
For example, can it choose its own destination for the dimension door as a separate location from the one chosen by the caster?
And can it bring its own willing creature along with it?