Question: That title is probably way too specific, but essentially I'm trying to develop my book's setting. Currently, my characters (a team of six adventurers) awaken as amnesiacs who were once human but now have been transformed into a motley menagerie, within a grassland. This hilly grassland has a road running through the center of it, and at one point, there lies a small mountain next to the road.

This mountain is rather important, as carved into its interior is a town named Rockhaven, where a plot-important SC (supporting character) awaits the team. What I don't know is what would cause a solitary mountain to form in the aforementioned biome-a grassland with rolling hills and plains-akin to the Kansan prairie. So, as you can see, my question is **What Could Explain A Solitary Mountain in a Grassland?**

Answer: 
Ancient meteor strike. Large meteor craters have flat bottoms (your plain) and a large central peak (caused by the ground directly under the impact rebounding).

See <https://en.m.wikipedia.org/wiki/Complex_crater> for some examples.
Just to give an alternative to (dead) volcanoes or volcanic intrusions:

**[Salt tectonics](https://en.wikipedia.org/wiki/Salt_tectonics)**

A salt dome may push a solitary rock or hill upwards, examples are the [Segeberger Kalkberg](https://en.wikipedia.org/wiki/Segeberger_Kalkberg) in Bad Segeberg, Germany, or the off-shore island [Heligoland](https://en.wikipedia.org/wiki/Heligoland) (imagine a lower sea level to place it in a grass plain).