The rocks were probably thrown out by the Ullysses Crater impact.
Especially one rock took our attention. It looks like the rock contains a part of the head of an animal that turned to stone.
1. Original image
Image 2 and 3: Enlarged
If you look carefully, right ear and eyes are clearly visible
Of course it may be pareidolia and it is just an oddly shaped rock, what you think?
Original Opportunity image: Sol 3778
Credit: Original color image is created by Holger Isenberg using filtered Pancam frames.