The Grass-type starting Pokémon for Pokémon Sun and Moon is Rowlet.
Rowlet is one of the three starter Pokémon offered at the beginning of the Alola adventure, alongside Fire-type Litten and Water-type Popplio. It is Pokémon #722 in the National Pokédex and was introduced in Generation VII, the generation represented by Sun and Moon.
Although Rowlet begins as a Grass/Flying type, its first evolution, Dartrix, keeps those types, while its final evolution, Decidueye, becomes Grass/Ghost. Rowlet is designed as a small owl-like Pokémon, fitting the Hawaiian-inspired Alola region. Its Pokédex descriptions highlight its ability to fly silently, attack with razor-sharp leaves, and rotate its head remarkably far.
A common mix-up is choosing Decidueye, the final evolved form, instead of Rowlet. The question asks for the starting Pokémon itself, not its evolution line.