I always try to avoid hitting any animal but you have to accept that sometimes you have no choice.
The largest I have killed was a fox that suddenly ran straight into my path. I was able to stop just beyond it, and moved the body onto the grass verge.
The most tragic was someone's pet cat that again just tried to dash across the road right in front of my car. At least, as with the fox, it died pretty well instantly, and again I moved it off the carriageway. Although there was no legal need to do so I reported it to the local Police Station (this was when there such things!) lest its owner should try asking there.
Driving along a very narrow lane one evening, I think I stopped and moved about half a dozen hedgehogs to safety! Luckily I did not pick up any parasites from them. On another incident I could not help herding a flock of sheep for nearly a mile along a lane bounded by banks and walls, until they all turned off into an open farm-yard. I stopped and called at the house to explain why they had a yard full of sheep that probably weren't even theirs.
