How to Unlock Mock Guide
Mock is one of the buggy Unlockable Monsters in MR2. You have to reach the 4th Trainer Rank early on in the game (by year 10 usually) and you can't go above the 4th Rank or Auntie at the item store might not give you the item you need. How this side quest usually works is as follows...
Go to Auntie's shop once you've reached the 4th Trainer Rank on a day that she isn't having a sale and she will give Colt a bag of seeds to plant at the ranch. This prompt is pretty rare so you may want to keep returning to the shop until it happens.
Once you get the seeds the next time you're at the Ranch Colt will plant them. Over the course of the next 10 years Colt will occasionally update you on how the tree is doing until one day the tree will bloom and not very long later it dies. After that Colt never updates you about the tree again, although you'll still see it on your Ranch in some shots.
The next time you have a Monster die on your Ranch, its spirit will inhabit the tree that died and you'll receive the Mock Monster. That's all there is to it!