Guts (known as Will in Monster Rancher 1) are the units of energy used in the majority of Monster Rancher games. Each attack a monster knows (called a technique) uses up a certain amount of Guts; for instance, in Monster Rancher DS, moves can cost anywhere from 20 to 60 Guts.
Both the player's monster and the enemy's monster start with 50 Guts at the beginning of a match, and replenish Guts when they are not attacking; a maximum of 99 Guts can be saved up. Monsters have different rates of Guts regeneration depending on their breed, their age, their frame of mind, and the game platform.
Some attacks, when they hit, deplete the enemy monster's Guts. Such attacks can be quite useful, as a lack of Guts translates directly to an inability to attack.
In earlier games, when you look at the information for a specific technique that your monster knows, it will specify the exact ammount of Guts needed to use that attack. However, as the algorithms got more complex in later games, the Guts needed became more unpredictable and thus was designated with a letter system of estimation, much like the Hit Rate and Damage of each move.
As an added feature, in Monster Farm DS and Monster Rancher DS, it costs extra Guts to push an opponent back into far range, and Guts cannot be collected while an opponent dashes forward into near range.
Guts are used in Monster Rancher Battle Card and the actual card games as well. Guts are collected by discarding the extra cards in your hand, so the strategy lies in keeping the card(s) you want to attack with while sacrificing some lesser cards for a chance to attack.