I'll get the ball rolling before people forget this thread exists.
Yertsi
Appearance: Frog/fish like humanoids covered in gills. Varieties of these creatures have been spotted in all of the colours of the rainbow. Humanoid appearance with average height of 7foot 2. Though this species has huge variation in height in adults from 2 foot to 9 foot.
What is known about them:
Little is known about this species, though most sightings of these creatures have been in the Veil.
They are magical creatures that are said to be somehow related to daemons.
Language: The Yertsi do not have a verbally spoken language. They speak telepathically with one another. Anything they communicate is automatically translated to whichever language the recipient speaks. The Yertsi never need to learn any languages and can communicate fluently with any creature including wild beasts.
Assumptions made about Yertsi: Yertsi are said to be gifted in mind reading, mind control and telekinesis. Though these rumours have been largely unfounded, the Tevinter Empire has an entire library dedicated towards these creatures. There are entire volumes about Yertsi using their mind control and wreaking havok. Due to many urban legends, when someone spots a Yertsi they either run from a Yertsi or try to kill the Yertsi on sight.
Do the Yertsi exist?: Some say the Yertsi exist. Some say they don't. Their very being remains a mystery to this very day.