This page which I'm making is a list of underleveled Pokémon that can be encountered in the wild or by trainers.
In the games, underleveled Pokémon can occasionally be found in the wild. Some trainers found early-middle of the game have underleveled Pokémon in their party.
In the games
Depending on the version and generation of the game, more or less underleveled Pokémon can be encountered. Prior to Generation V, Pokémon that are obtained in trades are always the same level as the Pokémon that was traded by the player. In some cases it is possible to receive an underleveled Pokémon by trading the requested Pokémon at a lower level than the natural level of when the Pokémon that will be received would usually evolve at. More can be seen here.
In Generation V, Pokémon Black and White Versions 2 the introduction of Easy Mode lowers the strengths and levels of in-game trainers Pokémon by a few levels. As a result, the decrease in level would lead to many trainers having underleveled Pokémon due to some of their Pokémon who evolve by level up being at the minimum level of evolution in normal mode.
In Generation VI, the introduction of the Friend Safari had also presented the opportunity to encounter many underleveled Pokémon due to the nature of the Friend Safari being that all the wild Pokémon that appear there will be level 30.
In Generation VII, the concept of SOS Battle's makes it possible for an evolved form of a Pokémon to appear alongside the pre-evolution of the Pokémon. This new type of battling along with all Pokémon that can appear within an area all sharing the same level range means that SOS battling can bring forth many underleveled Pokémon. For example, it is possible to catch a Salamence as low as level 9-12 due to appearing as an SOS ally of a Bagon in Alola Route 3 where the level range of all wild Pokémon are levels 9-12 within grass.
Totem Pokémon are also underleveled in the early to mid stages of the game due to the Totem commonly being an evolved Pokémon. However their underleveled status is due to its in-game level being lowered to be closer to the wild Pokémon that appears in the location of the trial.
Underleveled wild Pokémon
Underleveled Trainer Pokémon
|Name||Trainer||Game||Location||Minimum level||Level found||Notes||Picture|
|Dragonite||Lance||Gold, Silver, and Crystal
HeartGold and SoulSilver
|55||40, 47, 49, 50|
|Pidgeotto||Falkner||Gold, Silver, and Crystal
HeartGold and SoulSilver
|Violet City Gym||18||7, 9, 13|
|Lairon||Poké Maniac Steve||Ruby, Sapphire, and Emerald||Route 114||32||27, 29|
|Aggron||Poké Maniac Steve||Ruby, Sapphire, and Emerald||Route 114||42||35|
|Rhydon||Poké Maniac Steve||Ruby, Sapphire, and Emerald||Route 114||42||35|
|Purugly||Commander Mars||Diamond, Pearl, and Platinum||Valley Windworks||38||16DP, 17Pt||Only applies for the first battle at Valley Windworks.|
|Skuntank||Commander Jupiter||Diamond, Pearl, and Platinum||Team Galactic Eterna Building||34||20DP, 23Pt||Only applies for the first battle.|
Underleveled event Pokémon
|Artwork||Name||Generation||Country||Event||Minimum level||Level found||Notes||Date of distribution(D/M/Y)|
|Ash's Pikachu||VII||Japan||Cap Pikachu M20 Promotion||2||1||Original cap only. Distributed to players who preordered tickets for the 20th Pokémon movie in Japan.||Pre-film: 15/4/17-14/7/17|
|Dragonite||IV||Japan||Pokémon Center Strongest Pokémon Distribution Event||55||50||Local event at participating Pokémon Centers||15/1/08-14/2/08|
|Dragonite||IV||U.S. and Puerto Rico||Toys "R" US Dragonite||55||50||Local distribution event at participating Toys "R” Us stores||8/11/08-9/11/08|
|Lance's Dragonite||V||Japan||Pokémon Game Show Pokémon||55||50||Live distribution at Pokémon Game Show event in Tokyo||17/8/13-18/8/13|
|Tyranitar||VI||Korea||2014 Korean World Championship Series Pokémon||55||50||Live distribution for players who attended the World Championships 2014 Qualification Tournament at the Yeongdeungpo Lotte Department Store in Seoul, South Korea. Only works with Pokémon X version.||5/7/14|
|Tyranitar||VI||Japan||Pokémon Center Battle Championship Pokémon||55||50||Local distribution event at Pokemon Center stores in Japan||7/7/14-21/7/14|
|Ghetsis's Hydreigon||VI||Japan||Pokémon Center Occupation Plan Pokémon||64||52||Local distribution event at Pokemon Centers, Pokemon Stores and the Hokuriku Oyabe Pokemon Store Outlet.||1/10/16-11/11/16|