In the Pokémon series, most of them evolve when they reach a certain level or meet special conditions. And this is usually for the best - evolved forms of Pokémon almost always have higher stats and more powerful abilities. However, you can also cancel evolution and leave the Pokémon in a less evolved form. To do this, during evolution, you must quickly press the "B" button or use the Everstone.
Steps
Method 1 of 2: Cancel evolution with the "B" button

Step 1. Wait for your Pokemon to start evolving
Most Pokémon evolve after reaching a certain level. Pokémon level up by gaining experience points during battles. For different Pokémon, evolution starts at different levels.
- If you don't know when or how a Pokémon will evolve, you can easily find this information on the Internet. You just need to open the page of this Pokemon on a site dedicated to Pokemon, for example, on the poke-universe.
- Please note that some Pokémon evolve only with special items or when special conditions are met. For the evolution of the same Eevee, you cannot do without an evolutionary stone, no matter how many levels it lifts.

Step 2. Watch the evolution animation
When your Pokémon reaches a high enough level (or you use an evolution stone, etc.), you will see an evolution dialog box with the text “What? (pokemon name) is evolving! " or something similar. Wait for the evolution animation to start. The screen will start blinking, and the Pokemon will "jump" between its current form and the next evolution.
In modern Pokemon games, evolution screens are much more fanciful than in older games, but the general concept is the same for all games in the series

Step 3. Press button "B" quickly
As soon as you see the beginning of the animation, quickly press the "B" button. Just do not pull the rubber, otherwise the evolution will end.
Ultimately you should see a message with the text “What? (pokemon name) has stopped evolving! " or something similar. This means that evolution has been canceled. If your Pokemon level up, it will remain in the same form, but with a higher level

Step 4. Repeat the required number of times
After canceling the evolution once, you will not cancel it in the future. If your Pokémon starts evolving again, you will have to repeat the above procedure to cancel it. Otherwise, the Pokemon will evolve into the next form.
For Pokémon that evolve in a standard way as a result of pumping, the evolution will have to be canceled every time you level up
Method 2 of 2: Preventing Evolution with Everstone
Using the Everstone
If you don't have the Eternal Stone and don't know where to get one, click here.

Step 1. Obtain the Eternal Stone
This special item is used as a more effective way to prevent a Pokémon from evolving. As long as a Pokémon holds the Eternal Stone, it will not evolve, no matter what conditions are met for this.
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Everstones appear only in games of the second generation and older.
In other words, you won't be able to find them in games like Pokemon Red / Blue / Yellow.

Step 2. Let the Pokemon take the item
Open the "Items" menu and transfer the Eternal Stone to the Pokémon that should not evolve. A Pokémon can hold an item in the same way as a berry or similar item. As long as he holds the Eternal Stone, leveling up will not lead to evolution.

Step 3. Do not use evolution stones on the Pokémon
The Everstone action has one significant exception. Using an Evolution Stone on a Pokémon like Eevee, for which it is necessary for evolution, will still trigger evolution, whether it holds the Eternal Stone or not.
Search for the Everstone
Information about the location of items in the game is kindly provided by the Bulbapedia portal.

Step 1. Find out where to find the Everstones in the Gold / Silver / Crystal games
In these games, Everstones can be found by doing the following:
- Fighting wild Geoduds and Gravelers.
- Finished second in the Giotto National Park's beetle catching competition.
- After speaking with Bill's grandfather on Route 25.
- By showing Professor Elm how Togepi hatches from the Riddle Egg.

Step 2. Find out where to find the Everstones in Ruby / Sapphire / Emerald games
In these games, Everstones can be found in the following locations:
- Fighting wild Geoduds and Gravelers.
- On the second basement of Granite Cave (next to Route 106).

Step 3. Find out where to find the Everstones in the Fire Red / Leaf Green games
In these games, Everstones can be found in the following locations:
In the Mountain Tunnel (next to Route 10)

Step 4. Find out where to find the Everstones in the Diamond / Pearl / Platinum games
In these games, Everstones can be found in the following locations:
- Fighting wild Geoduds and Gravelers.
- By playing the Underground mini-game. Everstone is four squares wide and two squares high.

Step 5. Find out where to find the Everstones in the Heart Gold / Soul Silver games
In these games, the Everstones are located in the same locations as in the Gold / Silver games. In other words, Everstones can be found in the following locations:
- Fighting wild Geoduds and Gravelers.
- Finished second in the Giotto National Park's beetle catching competition.
- After speaking with Bill's grandfather on Route 25.
- By showing Professor Elm how Togepi hatches from the Riddle Egg.

Step 6. Find out where to find the Everstones in the Black / White games
In these games, Everstones can be found in the following locations:
- Fighting wild Roggenrols, Boldors and Gravelers.
- Spawns randomly in dust clouds (only found in caves).
- From a man on the second floor in the Castelia City Pokémon Center. To get an item from him, you must trade with at least 10 different people.

Step 7. Find out where to find the Everstones in Black 2 / White 2 games
In these games, Everstones can be found in the following locations:
- Spawns randomly in dust clouds (only found in caves).
- From the butler on the 23rd floor of the building south of the market in Driftvale.
- At the antique shop on Join Avenue.

Step 8. Find out where to find the Everstones in X / Y games
In these games, Everstones can be found in the following locations:
- Fighting wild Roggenrols, Boldors and Gravelers.
- After battling the psychic Inver on Route 18.
- Balloon Flapping Level 3 Prize at PokeMileage Club.
- From the Geosenge Town Scientist found in the house northwest of the Pokémon Center.

Step 9. Find out where to find the Everstones in the Omega Ruby / Alpha Sapphire games
In these games, Everstones can be found in the following locations:
- On the second basement of Granite Cave (next to Route 106).
- After fighting in the Inverse Battle Stop. You can win the Eternal Stone if you score between seven and nine points.
Advice
- Usually, the cancellation of evolution is used so that a Pokémon can learn a certain ability at a low level of its non-evolved form, and not after evolution. For example, Rattata can learn Super Fang at a lower level than Ratikate.
- In 3DS games, evolution can only be canceled after your Pokémon spins once during the evolution animation.