How to Unlock Metalner Guide
In order to unlock the Metalner Monster all you need to reach the 8th Trainer Rank and have at least the 2nd Barn and House upgrades. If you fulfill these requirements then randomly during the month of September, on a clear night, you'll be visited by Aliens. This entire quest happens automatically and you actually don't need to do anything the whole time, aside go through dialogue which is pretty funny.
Once you're visited by the Aliens you'll be visited again usually within the following weeks, assuming it doesn't rain. On the third visit you'll confront the aliens and they'll give you a Crystal which you'll want to take to the Monster Shrine in town.
That's all there is to it. When you arrive at the Monster Shrine the Crystal will dissipate and you'll be able to get the Metalner Monster off of any Discs that you use. Sadly you aren't given a Metalner, you'll have to go through some games/discs to get your hands on one.