I agree that dogs are a gift from God, but I don’t recall dogs having anything to do with Cain. Where is that in Scripture, please?
It is something I have heard repeated many times through my life, I am not a scholar on gospel, a part time student at best.
I have listened to elders and wingnuts talk from many different “theologies” there are certain stories that hold a basic “truth”
The gift of the dog is one of them.
This was a quick search, but, should send you in the general direction.
(I’m not buying the greyhound part..)
https://www.chaimbentorah.com/2014/10/hebrew-word-study-mark-cain/