Answering because I have further detail to add that helps elaborate.
Wynaut's type as the Bright Pokémon translates to the Merry Pokémon in Japanese.
Now, to find out what makes Wynaut the Merry Pokémon, let's look at some of its Pokédex entries. If we look at its Pokédex entry in Ruby, which was the game it was introduced in, it says
Wynaut can always be seen with a big, happy smile on its face. Look at its tail to determine if it is angry. When angered, this Pokémon will be slapping the ground with its tail.
Focus on the first sentence here and that shows why it is the Merry Pokémon, it's always smiling.