You'll get a notification for a Free Roam Event while you're out exploring the open world. Free Roam Events are often timed player-vs-player events, but these two are collaborative.
Red Dead Online Free Roam Events: Legendary Animal Protection and Wild Animal Tagging Details
Legendary Animal Protection is basically a payload mission, but you'll be protecting a Legendary Animal from poachers. Your Posse will be on horseback as you look for the caged animal. Take out the poachers and release it. From there, you must escort the creature to its final location while fighting off poachers. You just can't get too close to the animal, or else it will see you as a threat.
Wild Animal Tagging is a free-for-all tagging safari. You'll be let loose in an environment and be tasked with sedating and tagging as many animals as you can in 10 minutes. Tags you or your teammates get here will also count for everyone's Field Guide. Legendary Animals can also appear here, so it's a good way to rack up samples and entries.