I have been working on the same quest, Animal Extermination: Clear out Greenspring Hollow, for a few months now. Everytime I try to complete this quest there are no animals present. I know I already killed them but now there is no way for me to complete the quest. I do not keep old saves, I like to have only 1 recent save on my disk at a time. Is there any bug fix for this or a way to "reset" the animal?

Stay out of Greenspring Hollow for 31 days to allow it to reset. This also means stay out of the area and stay out of the HOLD it's in.
Every time you go near it, it will reset the respawn timer.

If after that time it still doesn't reset, then there's not much else you can do and no, you'll never complete the quest or be able to get another quest from the Companions.

Same goes for your thread about the Dragon.

I stayed away from the game for nearly 2 months... Are you saying i should go to a hold on the other side of the map and sleep for 31 in game days or go a hold on the other side of the map for 31 real life days?

In-game 31 days.

Ok, that is going to take a while to do. But i will get started on it now. Thank you very much Hanaisse.

I waited over 31 in game days and the animal that I need to kill in Greenspring Hollow did not reset, the 2 Mammoths and the 1 Giant did, but not the animal that I needed to kill. Didn't work with the Dragon at Mount Anthor either...