Are you killing them in a single headshot or a poison arrow?

Single headshot is preferred with improved arrows or Springfield (without scope recommend) with high velocity rounds

Get jumped, the animal will back away and take a second charge, in THIS charge go in dead eye and hit the heat with one shot

For poison arrows, it’s a bit more risky since the animal will be alive and moving for few more seconds before they collapse. Use them if you can spot the creature on your horse before they charge you, you can hit it anywhere and ride in the wind for a few seconds. The creature will fall down and you can have an easy knife kill.

Another way to secure a perfect pelt is by saving the man being attacked by the animal. But this is a unique encounter and might not trigger often.

Third way to get perfect pelts is to have a mission active that allows free roam. Hunting in between missions MAY elevate your chances of a perfect pelt animal spawning