The level 4 spell Polymorph requires concentration to maintain. The subject retains their personality and alignment, but they have the mental statistics of the new form, which must be a beast and cannot speak or cast spells.
If a caster targets themselves with this spell, can they concentrate to maintain it, or does this depend on the mental statistics of the beast form they choose?
This situation certainly occurs in practice, since Wizards reaching level 10 in the School of Transmutation gain the Shapechanger feature. This allows them to cast Polymorph, but only upon themselves and only to change into a beast of CR 1 or lower. Beasts with CR 1 or less range in Intelligence from the Giant Eagle (Int 8) to the Frog (Int 1).