Hitting a dog in a car, even if it runs off, is an accident which legally has to be reported to the police. Failing to report within 24hours is an offence - potentially a £5,000 fine!
Hopefully noone else reported it nor was it caught on CCTV or dashcam.
If you do report it to the police then it follows you must inform your insurance company.
So, was anyone looking or likely to report it?
I had to look into this a while back when a deer jumped into the side of the car. Hitting a deer is not an accident.