Anima is the name given to the life force energy of a living thing. It is similar to geonyte, in that it is a primal source of energy, but whereas geonyte originates in the earth, anima is generated by a living creature. Each creature (human, animal, monster) has its own unique anima. Unfortunately, in Monster Rancher EVO the denizens of the Warp Dimension build a robotic monster called the Anima-Eater, capable of drawing it out of living things.
Guts in the games is later referred to as Anima Points, because it is simply the storing up of a monster's anima to use in an attack.