Unnatural Selection

Unnatural Selection is a gameplay achievement and trophy in Red Dead Redemption in which the player must "kill one of every animal species in the game in any game mode."

Guide
It is not necessary to skin the animals. However, it is a good idea, because then you can track which animals you have killed in the Collectibles section of stats (it will show how many of each animal has been skinned).

List of Species
To unlock this achievement/trophy, the player must kill each of the following animals at least once:
 * 1) Armadillo
 * 2) Bear
 * 3) Beaver
 * 4) Bighorn Sheep
 * 5) Boar
 * 6) Bobcat
 * 7) Buffalo
 * 8) Cougar
 * 9) Coyote
 * 10) Crow
 * 11) Deer
 * 12) Duck
 * 13) Eagle
 * 14) Elk
 * 15) Fox
 * 16) Hawk
 * 17) Wild Horse
 * 18) Owl
 * 19) Rabbit
 * 20) Raccoon
 * 21) Seagull
 * 22) Skunk
 * 23) Snake
 * 24) Songbird
 * 25) Vulture
 * 26) Wolf

Trivia

 * The name of the achievement is a parody of Darwin's theory of natural selection. Players must seek out and eliminate animals, intervening in the proposed natural process.
 * While shooting an animal Marston will occasionally shout "I love natural selection!'."