Red Dead Redemption 2 legendary animals are dangerous and elusive creatures that will test your hunting skills if you try to find and kill them. There's more than just bragging rights as well, as you'll also claim rare pelts and other trophies once the hunt is completed, which can be taken to the Trapper in Saint Denis then converted into unique outfits and accessories.
Meeting the legendary bear near the beginning of Chapter 2 is your introduction to this challenge, but after that it's completely down to you to find the remaining animals as a fun (ish) side activity. So knowing where and when you can track them is valuable information.