Yes it evolves to slaking which is a super powerful pokemon.
Where you get slakoth in Pokemon ruby is in the petalburg woods. Trust me
lvl. 18 to Vigoroth lvl. 36 to Slaking
In Ruby/Sapphire/Emerald the chances of cathing a Slakoth is 5%. Just walk around Petalburg Woods for a while, you'll get one eventually.
Slakoth evolves into Vigoroth at level 18.
A slakoth evolves into vigoroth at level 18, then vigoroth evolves into slaking at at level 36.