There are two, or three depending on you look at it.
The first guards the Urn of Sacred Ashes, and you have likely already encountered it. You can get there by following the quest to obtain the Urn, including speaking with Weynon in Brother Genitivi's house and going to the village of Haven and going through the temple there.
The second is not really a High Dragon. If Morrigan's approval rating is high enough and you give the Black Grimoire found in the First Enchanter's Office of the Circle Tower to her, she will give you a quest to kill Flemeth. If you speak with Flemeth without Morrigan in your party and attempt to kill her, she will shapeshift into a High Dragon. The creature's title is "Flemeth the Shapeshifter", but it's exactly like a High Dragon.
The third is the Archdemon, which is a corrupted High Dragon. It's significantly more powerful than a normal High Dragon and can do some more tricks, too, so while in lore it is a High Dragon game-wise it really isn't.