While mounted on your steed, you can make any spell you cast that targets only you also target your steed.
The Wish spell (PHB, p. 288-289) has a range of Self.
What happens when someone mounted on his Found (Greater) Steed casts Wish?
- Only the caster will benefit from it, since Wish has the potential to affect more than just you (especially if you use it to replicate an existing spell)
- Both the caster and the steed will benefit from it, but it will have to be the same Wish effect (ex: both doing the 25 000 gp value object appearance)
- Both the caster and the steed will benefit from it, and will both be able to choose their own effect (ex: the caster replicating spell X and the steed replicating spell Y). Sub-case here is whether the "targets only you" restriction must also apply to the chosen spell(s) or not.
- Other behaviour
One crazy scenario I can think of if #3 is true: sitting on a Steed and casting Wish. The Wish you choose is a Simulacrum of you. The Wish the steed chooses is also a Simulacrum of you. You now have a simulacrum controlled by you, and a simulacrum controlled by your steed controlled by you. Absolute bollocks.
Another crazy one: The Wish you choose is [something unimportant]. The Wish your steed chooses is Find (Greater) Steed, so that your steed... controls a steed... And then, repeat it the next day, while the first steed is reduced to mount the second steed (and you are still small enough to mount the first steed), so that you cast Wish for [something unimportant], so that your steed gets Wish to cast [something unimportant], and your steed's steed gets Wish to cast Find (Greater) Steed... Creating a Command chain of Caster -> Steed -> Steed -> Steed. Absolute bollocks!