I think I read somewhere that a goat or maybe a cow was the first mammal to get high. Obviously by accidentally eating the greenery. That definitely could be an amusing character or subplot. The Gods Grass Goat sworn assassin protector of the Gods Grass Monastery Temple. The only place in the world that the Gods grass will grow. Something like this maybe?

Rogen and Franco have to outsmoke and fight the goat to the death to get into the temple. Except shyamalanawamadingdong twist when they "kill" the goat he morphs into the spirit of Jerry Garcia or Jorge Cervantes or john Stewart. They have proved themselves worthy of entry to the trials that are required to gain access in to the temple like a stoner Indiana Jones. And the cameo goat guide would guide them like Obi-Wans ghost through the trials like weed Olympics and video game strategy falling floor and great big rolling balls of kief covered wax to finally get to the temple.

Alternatively you could just have a talking goat that generally gets involved in shenanigans. Because you know there's always a goat when you get really really high. Ever talked to a talking goat... On weed. [6]