Clamperl was first introduced in Pokémon Go as part of a special Limited Research event in February 2019.
Upon release, it was possible to immediately evolve it into two Clamperl evolutions - Huntail and Gorebyss - though the game puts a literal question mark over what it evolves into.
How to evolve Clamperl into Huntail and Gorebyss
Clamperl can evolve into Huntail and Gorebyss, and getting both will be by chance because Clamperl evolutions are random.
With this in mind, what's the most practical way to approach this? Obviously the first evolution won't matter - it's going to be something you haven't had before - so it's a case of fingers crossed the second will be the other evolution you need.
If you get a duplicate, remember there's the trade feature - find someone else with a duplicate of the other and voila, the Pokédex entry is yours.
Remember if you don't get the Clamperl Candy you need, then Clamperl wasn't exclusive to the Limited Research event in debuted with - it'll be available in the wild afterward, albeit in uncommon quantities.
Alternatively, have Clamperl as your Buddy until you have enough to evolve again.
Pokémon Go in 2020 begins with the new Buddy Adventure feature, which introduces a variety of perks - including finding Souvenirs - as well as a new trade evolution feature. 2019 saw the release of the first Gen 5 Pokémon, along with new regional exclusive Pokémon, Egg charts, and the introduction of the Unova Stone. Last year also introduced the Adventure Sync rewards, a Jirachi quest, Team Rocket Invasions and freeing Shadow Pokémon, as well as monthly Field Research rewards. Complete The Take-Over Continues special research quest line to fight against the invading Team Go Rocket Leaders and Giovanni! Finally, we have details on the next Pokémon Go Community Day.
How does evolving Clamperl into Huntail and Gorebyss work in the main games?
Clamperl evolves into Huntail and Gorebyss in the main Pokémon series via held-item trading, much the same as for Gen 2 Pokémon like Steelix, Politoed, and Kingdra.
Specifically, holding a Deepsea Tooth for Huntail and a Deepsea Scale for Gorebyss are needed.
However, unlike many other item creatures in Pokémon Go, Clamperl doesn't need to hold an item at all for this to work, instead making the evolution random.
This is likely to keep the item pool as low as possible. This decision would also explain why so many Gen 4 creatures simply need the Sinnoh Stone, instead of the various items or methods required in the main games for those respective creatures.