I have seen Coyotes in Fullerton Loop by Blair Witch, I got scared on Thursday evening ride cause one ran out besides me so I just stopped and tried taking out my phone to take a pic but it ran into the bushes really quick. Also saw a turkey on the Floop by the Dam during Tour de Trypthophan. Plenty of suicidal bunnies on the floop!
One time I was riding in Chino Hills with 2 buddies who were new to MTBiking, we saw something big and yellow with a black tail coming down a hill, at first we thought it was a huge dog . It stopped when it saw us and then crouched down, my buddy was like it's a mountain lion!! We slowly backed up and then turned around and started riding as fast as we could, I kept thinking maybe that was a bad idea. I have also seen bobcats and tarantulas up in Chino Hills.
We saw a road runner a couple of times at Bonelli Park, I remember Vince The Trail Slayer was keeping up with it going on the downhill right before you get to the airport.
Never seen any snakes on the trails so I'm kind of glad but I'm sure I will someday.
I have also seen deer in Marshall Canyon and El Moro.