How does the Dimension Door spell interact with a mount summoned by the Find Steed or Find Greater Steed spells?

Question

Find Steed has this sentence (and Find Greater Steed has an equivalent one):

While mounted on your steed, you can make any spell you cast that targets only you also target your steed.

Dimension Door has this restriction with regard to taking someone else with you, where the restriction is size is the important bit:

You can also bring one willing creature of your size or smaller who is carrying gear up to its carrying capacity.

Take a Medium-sized character (such as a Bard with Magical Secrets) mounted on a Large mount summoned by such a spell (such as a warhorse), casting Dimension Door.
Is it possible to make Dimension Door affect this mount?

Answer 1

So long as you only target yourself you can take the mount.

The crux of the question is whether Dimension Door qualifies as a spell that "targets only you" as required by Find Steed.

There is an argument that Dimension Door targets a point in space and so would not qualify. I suggest, however, that interpreting the "spot" mentioned in the spell as a target does not match how targets are discussed in the rules.

The rules for targeting mention targeting a point in space only in reference to the point of origin for an Area of Effect:

For a spell like fireball, the target is the point in space where the ball of fire erupts.

A spell's description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect

There is no indication that a space can be a target except as the point of origin for an area of effect. The second quote indicates that there are three types of targets: creatures, objects, and points of origins for AOEs.

Unfortunately the description of Dimension Door makes no explicit mention of "targets" but we can infer from the text that you and whichever creatures you take with you qualify as targeted creatures, fitting the first category of "target". Furthermore the spell does not describe the point of origin for an area of effect.

If the Bard wanted to take another person with them when using Dimension Door then they would be unable to take their mount because the spell wouldn't target only them.

If, however, the bard wanted to teleport just themself, without taking an additional creature as allowed by the spell, then the rules for Find Steed would allow them to also bring the mount.

If a DM thinks that the space is also being targeted and so disallows Dimension Door you could always take the Mastiff, a medium mount instead considering that RAW it can still serve as your mount.

Answer 2

Dimension door targets a point in space.

Dimension door has a range of 500 feet, and states:

You teleport yourself from your current location to any other spot within range.

The rules for spell ranges state:

The target of a spell must be within the spell's range.

And the rules for targets state:

A typical spell requires you to pick one or more targets to be affected by the spell's magic.

In addition to targeting the caster, as well as possibly another creature, dimension door also targets a “spot within range”, so it does not interact with the found steed’s feature that allows a spell that targets only you to also target the steed - dimension door does not target “only you”.

Another feature that states "targets one creature" was erratad to mean "incapable of targeting more than one creature".

Consider the sorcerer's Twinned Spell metamagic:

When you cast a spell that targets only one creature

This is quite similar to find steed's requirement:

you can make any spell you cast that targets only you

However, Twinned Spell was later given an errata so that it only works when the spell is incapable of targeting more than one creature:

To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level.

Extending this ruling to dimension door seems quite natural, as the language is the same. "Targets only" has been officially ruled to mean "incapable of targeting anything other than", ergo "targets only you" means "incapable of targeting anything other than you". And finally, as we see in the description of dimension door, it is capable of targeting another creature other than you:

You can also bring one willing creature of your size or smaller who is carrying gear up to its carrying capacity.

Your mileage may vary on how compelling you find this particular observation, since dimension door was not given a similar errata.