Well, that’s not because of the mountain. Allow me to explain. Y’see, she’s not just a yeti… she’s a scary yeti. Scary is an adjective. An adjective tells you more about a noun or pronoun.

As for NA adjectives, I'll explain with an adjective, JÔZU (to be good at) as an example. You've learned that when a NA adjective modifies a noun, it takes NA at its end. So, JÔZU becomes JÔZUNA.