In the Monster Rancher series, your monster doesn’t live forever. Whenever your monster reaches the end of its lifespan, your assistant warns you and tells you to make a decision on how it will spend its remaining days. There are a few different choices you can make: letting your monster die from old age, selling it to the market to get some money, or freezing it. Out of the three options, if you want to get a better monster, freezing it is the best way to go.
Heading to the Lab, you’ll have the option to freeze your monster. Freezing it has no impact on its wellbeing so if you decide to unfreeze it, you don’t need to worry about its mood being negatively changed or it disliking you for your decision. If you do decide to keep your monster frozen, that opens up the option of combining it with another frozen monster. Doing so will result in a new monster. In both Monster Rancher games, it costs 500G to combine.
When you select your two monsters, you’ll be asked if you want to use an item. Using an item during combining could increase the chances of getting a particular result. Once you proceed with the fusion, you’ll have the created offspring that takes features from both its parents such as their traits and stats.