I've been cutting things around here for maybe 20 years. Never seen a fawn in my way (never even entered my mind, I'm glad to read about it so I might be more observant)
I did once however, chop up what I think was a reasonably large black snake.... (or a snake of some kind)....all I really know is all the sudden, the 5' mower hit something and bits of snake came raining down on me as it went flying everywhere.
Sure would hate to run over a fawn.... my wife would go apoplectic if that happened.
Using a 15' mower now instead of the old 5'. Usually keep an eye open on one side to keep the cut line going... if there was something laying on the other side, I'm guessing might be easy to miss.