All Pokémon, that to evolve into them there is evolution item required, cannot be neither caught in the wild, hatched from an egg nor be a Raid Boss in the gym and the only way to acquire them is the evolution.
Historically, in the Pokémon game core-series, the following conditions described below will take place when one of these items are given to a Pokémon to hold:
Pokémon capable of holding either a Dragon Scale, King's Rock or Metal Coat will evolve when traded.
Pokémon capable of holding either a Sun Stone or Up-Grade will evolve when leveled-up.
On the official support page for Pokémon GO, Deepsea Scale and Deepsea Tooth are mentioned.
In the main series games, these items are required to evolve Clamperl into either Huntail or Gorebyss respectively.[1]