Not all Pokémon rush towards you in the Pokémon Sword and Shield grass. Some of them take a while to locate and you have to go to great lengths to add them to your location. For example, if you want to add Feebas and its Milotic evolution to your Pokédex, you need to spend a lot of time fishing.
You can find Feebas in Pokémon Sword and Shield, but you don't have the best chance. She shows up on Route 2 and you need to throw the fishing rod into the water along the route to wind her up and fight her. Unfortunately, you only have a one percent chance of catching him on this route. There are ideal fishing spots on this route west of the Professor's house, and to reach them you need to use the Rotom Bicycle upgrade to cross the water.
This percentage probability means that you need to spend a lot of time in a water hole along this path to find it. Since it has a low chance, it spawns between levels 37 and 40, so it's a decent battle. Feebas is a pure water type, so you may want to avoid using the electric and grass types to fight it, to make sure you don't pass out. After capturing it, turning it into a Milotic requires some work.
To add Milotic to your Pokédex. You have the option to find a prismatic scale and have Feebas held in your hand. While holding the item, exchange it with a friend of yours and it should evolve into Milotic. I hope your friend has the good faith to offer it to you. You should have it in a short time and added to your collection and list.
Milotic cannot be spotted in the water. You have to exchange it for the prism scale.
Given how long this process can take, locating a Prismatic Stone and trading it with a friend seems like your best bet for adding Milotic to your Pokédex.