My thoughts on symbols of Fenris. As well as other things I feel represent the great wolf. These are my feelings and thoughts. They are not verified by anyone. These are personal to me. If they work for you great.
Wolf– which is a no brainier. His first form is of a wolf. He is the alpha as well as the lone wolf. For until his boys he is his own pack. With his boys he is dominate the alpha wolf.
Sudden geological events– He is the sleeping volcano that suddenly erupts with fury. He is the earth shaking as the plates slip. He is the earth splitting apart.
Dog– He is loyal to those that love him. The dog is a more seemingly tamed Fenris. One that appears to be tamed but at any moment could react in a way that is not as a tamed dog should.
Broken chains– Chains kept him down broken chains represent the freedom he won by fighting back.
Destroyed human made things– He is the destruction part of the Nature cycle. Though destruction rebirth comes.
Tears– He is the tears of rage and frustration shed.
Emotional upheaval– The emotions that churn inside waiting for a moment to break free, that is also Fenris.
Natural events that destroy (that are not weather related)– Mud slides, rock slides ect. Natural events that destroy but are not weather related.
There is more I am sure but That is what I feel and think for the moment.