Move: Trick Room, as an Egg Move from the Slowpoke family.
Stats and Typing: Marowak's stats seem only half-decent at a glance, with the only stat crossing base 100 being its nifty base 110 Defense. However, Marowak boasts exclusive access to the powerful Thick Club item, which doubles Marowak's Attack stat when held; this effectively allows it to achieve a frightening 568 assuming an Adamant nature. Unfortunately, its pitiable base 45 Speed stat limits its usage only to Trick Room teams in the higher tiers. As this often requires support on the behalf of another Pokemon, Marowak being able to manually set up Trick Room would be a godsend as it has both the bulk and typing to set up and sweep almost flawlessly.
Ability: Marowak has access to a good ability in Rock Head. While most of its moves fail to benefit from it, Rock Head allows Marowak to abuse a recoil-less Double Edge in place of the less powerful Return. It also has Lightningrod and Battle Armor, but those are generally inferior choices compared to Rock Head.
Design: Trick Room is widely distributed amongst Ghost-types: and while Marowak is neither a Ghost-type himself nor inherits Trick Room from a Ghost-type Pokemon, Marowak's association with the dead and Ghost-types is very strong as seen in RBY's Lavender Town chapter.
Need: Marowak excels in Trick Room, allowing to abuse a gargantuan 568 attack stat while technically "outspeeding" many other Pokemon in Trick Room due to its shitty 45 base Speed. Being able to manually set up Trick Room would eliminate the need for Trick Room support from another Pokemon. In short, Marowak needs a way to mitigate its bad Speed stat in order to successfully abuse its massive Attack stat, and Trick Room fits the bill perfectly.