The character 'dragon' form was deliberately designed as a wyvern, to better correspond to the human form (ie arms become wings, rather than wings just sprouting out of the character's back, or something).
I don't know if Patriarch took a wyvern form to talk to your character, because he wanted a change of pace when he moved to that cave, or Larian just went with a different design. He had arms, legs and wings in Divine Divinity.
