They then cast Polymorph on themselves and turn themselves into a T-Rex, because they are cool.
Find [Greater] Steed says
While mounted on it, you can make any spell you cast that targets only you also target the mount.
So Polymorph can affect both targets. However, since the steed is less than CR7 what happens?
- Both bard and steed both become a T-Rex
- Bard becomes a T-Rex and has to select a different form for the steed according to it's CR
- The bard becomes a T-Rex but the spell fails on the steed
- The spell fails entirely because the steed isn't a valid target to become a T-Rex
- The bard and steed become some kind of conjoined T-Rex abomination
If there is a difference between the spells, please use Find Greater Steed as the spell I care about, and assume the mount is a Griffon.