Would like to know what pratikriya (remedy) should we do if we accidentally kill an animal e.g., a car hitting a cow or deer accidentally or car running over a snake?
Also, in the case of a vehicle killing an animal should we change the vehicle?
The Smritis give us penances for all sorts of sins committed.Some even give you penances for accidentally killing animals.But many of these penances will look outdated or at least will be difficult to perform for someone living in this age.
In any case here are a few of them:
By [consciously] killing a frog, mongoose, crow, cat, boar, mouse or a dog, a twice-born person should perform a great penance extending over sixteen days
By [unconsciously] killing a dog, one should zealously drink milk for three nights. (7)
by [unknowingly] killing a cat or a mongoose, one should wend a way a Yojan [in length]. By killing a horse, a twice-born person should perform a hard austerity extending over twelve [days]. (8)
To take clarified butter is the purification [enjoined for destroying] flowering [plants]. By killing, by mistake, [a cow], one should perform the Chandrayana* and the Paraka.
Usana Smriti,Chapter 9.
BTW,Usana Smriti refers to the Smriti composed by Daityaguru Sukra and is one of the major 18 Smritis.
So,for accidentally killing a dog the remedy seems to be living on milk only for three nights.