This question asks whether a creature summoned by Find Steed or Find Greater Steed can be brought with the caster when the caster uses Dimension Door.
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?
Best Answer
Assuming as the question does that you can use dimension door like this with a found steed:1
Dimension door targets the steed in the same way that it targets the caster.
Find (greater) steed states:
The obvious application of this feature is this: the spell targets the steed in the same way that it targets the caster. Which is:
So both the caster, and the steed, are subject to this effect, and all of the effects that follow in the spell description. To put it another way, once you have chosen to have dimension door also target the steed, the effect is resolved as if the steed had also cast dimension door at the same time. The entire spell description applies independently to the caster and the steed (though for the purposes of features like counterspell, the steed is not treated as having cast the spell).
1 However, I cannot help but shamelessly plug my objection to this assumption.