A dragon can cast spells with a somatic elements. If the dragon is polymorphed with baleful polymorph into a mouse and passes the second saving throw retaining the ability to cast spells:
will he still be able to cast spells with the somatic component?
will he still be able to cast spells with the verbal component?
A mouse cannot speak and is not really similar to a dragon so I would tend to assume that both answers are a no but then I would not see why the ability to cast spells is retained since it would be rarely the case that someone balefully polymorphs a creature in a similar creature able to speak ...