The motto of the Pokémon Sword and Shield to only part of the Pokédex, it hasn't actually been possible to acquire every Pokémon in-game since the very original titles.
While most Pokémon evolve by leveling up, several monsters need to be traded before they change into their final form. This can be frustrating for players that don't want to work with others, but, in some cases, the result is strong enough to make the hassle worth it.
Gengar
Even eight generations after its debut in Pokémon Red and Blue, Gengar is still one of the coolest Ghost-type Pokémon in the franchise. It's gone through several buffs and tweaks that have come and gone, including the addition of the Levitate ability (which it lost in Gen VII) and a fantastic Mega Evolution (a unique feature of Gen VI), but this spooky specter is still capable of competitive viability.
Its Speed and Special Attack stats are some of the best in class, and its Gigantamax form lets it use G-Max Terror. Fully-evolved Gengars can be caught in the Wild Area, but Haunters only evolve via trade.
Scizor
Scyther was one of the first Pokémon to receive an evolution in a subsequent generation, and Scizor quickly became a fan-favorite that has bounced in and out of competitive viability.
One of the unique aspects of Scyther and Scizor is that, unlike most evolutionary chains, they both have the same total base stats. Scizor simply trades some of Scyther's Speed for higher Attack and Defense and becomes Bug/Steel instead of Bug/Flying. Trainers can obtain a Scizor if they trade a Scyther while it's holding a Metal Coat.
Slurpuff
Slurpuff's dessert-inspired design may make it one of the most underrated Fairy Pokémon in the franchise, but this pink pocket monster is a terrifying offensive powerhouse with the proper move set. Its viability hinges on using Belly Drum.
This rare move makes the lose 50% of its max Hit Points in exchange for maximizing its Attack stat (four times standard damage). While this puts Slurpuff closer to a knock-out, berries can mitigate the damage. When Slurpuff is buffed, it becomes capable of taking down many foes in a single hit.
Politoed
Politoed may seem a bit out of place compared to the unified blue and white color scheme of Poliwag, Poliwhirl, and Poliwrath, but this green, froglike Pokémon is a fantastic addition to specific weather teams.
Politoed has the ability Drizzle, which summons rain every time the 'mon enters the battlefield. Rain teams want the rain effect up during as much of the battle as possible, so being able to summon it without wasting a turn to bring it on via a move can help these teams keep their momentum.
Milotic
In the first generation of Pokémon games, Magicarp and Gyarados were unique for their gimmick of a comically-weak monster evolving into a powerful one. Game Freak has attempted to emulate this a few times since, including with Feebas and Milotic.
Competitively, Milotic is a solid Water-type tank that can take hits from most foes. Its ability, Marvel Scale, also lets it benefit from improved Defense when suffering a Status Condition. This Pokémon is unique in that it was originally evolved via raising Feebas' Beautiful condition, but it has been trade-only since Generation V.
Conkeldurr
Conkeldurr is a Stage 2 Pokémon first introduced in Generation V that evolves from Timburr and Gurdurr. While Conkeldurrs can be caught in fully evolved in the Wild Area, trade is the only way to evolve a Gurdurr. As its appearance suggests, Conkeldurr is a purely offensive Pokemon with access to several powerful abilities.
Guts is the most commonly used; this ability ignores the damage reduction from Burn and multiplies Conkeldurr's attack by 1.5 whenever it's suffering a status condition. This is often paired with a Flame Orb to land devastating Facades.
Trevenant
Trevenant, also known as the Elder Tree Pokémon, is a Ghost and Grass-type monster that excels at dealing damage with its access to numerous strong STAB attacks. While Trevenant may be one of the ugliest Grass-type Pokémon in the games, it's one that more trainers should consider using.
Poltergeist and Wood Hammer both deal substantial physical damage, and moves like Horn Leech can levy Trevenant's high Attack stat to heal it back up when its HP gets too low. Trevenant evolves from Phantump upon trade.
Machamp
Machamp was one of the original Fighting-type Pokémon introduced in Pokémon Red and Blue. While it's not usually seen much competitive use, it was given a Gigantamax form in Pokémon Sword and Shield that makes it much more intimidating.
Gigantamax Machamp gains access to G-Max Chi Strike. This move raises the critical hit chance of Machamp, and this effect can stack via multiple uses (and with Focus Energy). Also unique is that it can be either a Physical or Special attack depending on the move it's based on. Machokes evolve into Machamps when they are traded.
Steeelix
Steelix, the Iron Snake Pokémon, is a colossal monster that was introduced to the franchise in Generation 2. To get a Steelix, players must trade an Onix that is holding a Metal Coat to another player.
Steelix has one of the highest Defense stats in the Pokémon franchise at 200, and Steel's many resistances further boost this bulk. Players often use Steelix as an opening monster, as it has access to Stealth Rock, and Sturdy ensures that a bad initial match-up won't knock out Steelix.
Alakazam
Alakazam is a Psychic-type Pokémon that evolves from Kadabra via trade and is famous for its over-the-top Pokédex entries. According to the Pokédex, Alakazam's brain is more intelligent than a supercomputer, and its IQ is said to be 5,000.
It is also said to everything from the moment it's born. Alakazam isn't currently as intimidating on the battlefield as it was when it could Mega Evolve, but it's still a solid Special Attacker with great Speed. Alakazam is often played with a Focus Band to let it survive one hit as it self-buffs or attacks.
