You assume the form of a different creature for the duration. The new form can be of any creature with a challenge rating equal to your level or lower. The creature can't be a construct or an undead, and you must have seen the sort of creature at least once. You transform into an average example of that creature, one without any class levels or the Spellcasting trait. […]
The spell turns you into an "average example" of the chosen creature. But you also must have seen the "sort" of creature at least once. So can you turn into a unique or named creature, where the "average example" is basically one and only one thing and the only "sort" of creature is similarly that exact creature.
A somewhat related question: