Can Polymorph turn a character into a dragon?

This first sentence in the “Polymorph” spell description says

This spell transforms a creature with at least 1 hit point that you can see within range into a new form.

I have heard that polymorph only turns into “Beast”, but one of my PC’s pointed out the “new form” part claiming that a level 7 Wizard could turn a level 6 Paladin into a young white dragon (challenge rating 6).

This doesn’t sound right to me because I think it’s only beasts, maybe it’s a druid thing? Can someone please clarify is “new form” mean “any creature or monster” or “any beasts”?