Breeding

Description
Looking at the Momochi breeding chart, you can see that breeding Momochi with the same rarity has a chance to yield higher rarity ones and higher rarity Momochi will look really different compared to those that you will find in the wild. In this example, the horn color, coat pattern and coat color are different in each rarity and they will look better as the rarity gets higher. You can only find common rarity Magnus in the wild so any higher rarity Magnus would need to be bred.

Reference

 * Kickstarter Update #29 Magnus Breeding Stretch Goal Fish Caught & Final Boss Stretch Goal Fish Revealed!