guerrilla, gorilla

Guerrilla or guerrilla ( ger ill uh ) means a soldier not attached to a regular army, or one who fights in the forests or streets, attacking enemy soldiers wherever they are found--
The guerrilla hid in the mountains, waiting for the troops to get within shooting range.
The army was harried by guerrilla attacks all the way back to the camp.

Gorilla or gorilla ( gur ill uh ) means the large ape found only in Africa--
Dian Fossey learned to communicate with gorillas.
The gorilla looked out of place in the cage at the zoo.

Now that you know that, you can say--
"The gorilla was frightened by a guerrilla with a rifle, and hurried into the forest."

Note--a "guerrilla" soldier may not be the same as an "irregular" or a "partisan", although an author may sometimes use these interchangeably--
An "irregular" is something more like a member of a special forces unit--
He worked out every day, hoping to be assigned to an elite unit of irregulars.
A "partisan" is a citizen of an occupied territory who sympathizes with or aids a band of revolutionaries, who may use the tactics of guerrillas--
They were given the address of a partisan who had offered to hide them from the occupying troops.