Everyone in the mission will get a sample of the legendary animal. It's unclear if this is guaranteed, but it happened in my experience. During the mission, a legendary animal can spawn.

Wild Animal Tagging: You and a group of players cooperate to sedate and sample a bunch of normal animals.

Everyone in the mission will get a sample after it ends.

Protect Legendary Animal: You and a group of fellow players rescue an animal and then fend off poachers while escorting it to its destination.

There are a couple of new events in which you can participate to net you a legendary animal sample: Just remember: Harriet doesn't like it when you kill animals. The mission ends as soon as you either skin or sample the animal. These missions will eventually kick you if you fail to find the animal in the time limit, though unfortunately you only get a 30-second warning. Each animal mission has a couple of variants, so it might be different the first few times you attempt it.

Legendary animal missions are somewhat similar to legendary bounties in that they'll have you tracking a target across an area and sometimes fighting enemies while you do it. She usually has two or three missions available at a time, which each have a countdown timer showing when a new mission will replace it, usually between 30 minutes and an hour. One of each legendary animal species can only be found during their respective missions given by Harriet after you reach level 5 in the Naturalist role.

Rdr online interactive map