Dragons were rational and could speak (all the dragons Tolkien describes in detail were highly prolix). None of the beasts is recorded as acting in a rational manner or speaking - they were beasts not "Rational incarnates."

Tolkien is not shy of calling a dragon a dragon. Indeed he has a number of more detailed descriptive names for them, cold-drakes and fire-drakes for example. Tolkien never describes the beasts as dragons.