Each animal in Mope.io has its own outline, that determines its place the food chain. Animals with a green outline are prey; you can eat them. However, prey is also able to tail-bite you, a phenomenon that will mean you receive damage and lose some XP. Due to this, the tails of predators will have a green outline, but they are often hard to get to. See the main article for more info. If an animal has a red outline, it is a predator and is able to eat you. Like with prey, you can tail-bite predators too.
There are some animals that are either too high on the food chain to consume you, or too low a tier for you to eat them. They will have a dark green outline. Note that these animals can still cause you to be affected by certain Special Abilities, but the effect may not be as severe/harmful.