Varterral

Striders are huge spider-like creatures that stand on four massive legs and move with surprising speed. They spit corrosive poison and are able to leap after prey.

Background
These creatures are deadly guardians, often associated with the lost, ancient elven culture of Arlathan and the times before that the elves have lost to history.

Witch Hunt
A Strider appears in the trailer for Witch Hunt, the final DLC for Dragon Age: Origins.

Dragon Age 2
The Strider is the first new creature for Dragon Age 2 to be described on Bioware's site.