From the extended preview it looks like Emma goes down the elevator in the mine shaft and confronts a dragon which could be Maleficent’s “surprising counterpart” in SB. Maybe that is her true form as everyone seems to be in their true forms in SB. We know from the Disney version of Sleeping Beauty that it was at least one of her forms.
That is a possibility. There are stories of dragons who take human form, mostly from China and other Asian countries, along with the phoenix and other mythic creatures. European dragons tend to just be giant lizards with a propensity for hoarding treasure and burning villages.
Or perhaps, if Emma is fighting a dragon, then Regina somehow locked Maleficent in her dragon form. I want to know why Emma would be fighting a dragon in the first place. There must be something that is needed to break the curse or save Henry and the dragon or whatever is guarding it.