Just found this combo suggested in another thread and there were no comments about whether it works RAW. The idea is as follows:

Find Steed /Find Greater Steed specify that any spell that only affects you while mounted on your steed can also affect your steed.

Shapechange doesn’t function based on CR like true polymorph so when you cast Shapechange on yourself, it affects your steed as well and “The new form can be any creature with CR equal to your level or lower.”

This suggests that both you and your mount can Shapechange into dragons. So a wizard or Bard uses Wish to get a Pegasus, and the next day can flood any battlefield with TWO creatures that would take most of the party to kill.

Are there any limitations to this that I can’t see right off the bat? I’m assuming your mount retains its alignment and mental stats, just like you do. I guess one question is if you maintain your mental link with the mount, and if it’s still guaranteed to be friendly to you? Are there any other ways this could go haywire for the caster?