I'm a beginner at modding but I did run through some tutorials to try to make sure I was doing this right.
After looking at animal mods, I decided to load up the creation kit to try to change the factions on Sabre cats to make them passive to the player. I made the skyrim.esm active, added the player faction to the sabre cats, verfied that affected the sub groups, saved as a new esp, and loaded it up with the game. No other changes.
Well, the long and the short of it is, sometimes it works, sometimes it doesn't. Some characters I load are ignored by Sabre Cats, others attacked immediately. I've non-fast traveled across the map (to make sure I was in a different cell), waited (but not 31 days, because, sheesh)... basically 2 out of 5 characters are ignored, the other three are attacked.
What did I miss, what else should I do? Is there some characteristic of one character that can be different from others that makes actors see them as "not players?"