I'm writing this out of grief
I had animals outside my base while I was waiting for them to be tamed and a pack of wolves slaughtered all of them, I now know not to do that again but I still want to carry on their would be legacy for more than just a runestone.
Possibly something where animals leave skulls with their bones which can either be studied or put into a lesser grave (only requiring 1 or 0 beautiful dreams)
I can't think of a benefit, but if there was it should probably only apply to tamed animals so people don't just wildly kill animals for some arbitrary benefit. I would suggest something like ancestor worship but for animals cause like fuck it, why not. if you breed the animal and then one of the parents is killed or dies (possibly an aging/ disease mechanic but that could just be more annoying) you can then bury its skull for like a 5% increase in some quality or skill or something like that.
or it could just be nothing and be purely aesthetic where the skull would be on the grave because I am very sad rn.