Rodents

Opening Sequence
A rat appears inside the broken TV set in the very beginning of the opening sequence.

Freak City
When Jake lies down in a pile of trash, a number of rats ran up to him and started crawling all over him, which Jake didn't seem to mind at all.

City of Thieves
There was a rat in the episode, "City of Thieves" who had its cheese stolen.

Too Young
Candy Corn Rats are seen in the episode, "Too Young."

Memory of a Memory
A rat could have possibly lived in the Tree Fort, because in "Memory of a Memory," a tiny Finn and Jake came out of a mousehole into the memory of Ash and Marceline's break-up.

From Bad to Worse
Science is Princess Bubblegum's lab rat. It is trained to read her notes and gather ingredients for chemical formulas. Princess Bubblegum awards Science the Royal Medal for Heroic Bravery for creating the formula that reverses the Candy People's zombification.

Holly Jolly Secrets Part I
A rat was briefly seen in the episode "Holly Jolly Secrets Part I'" where it was sen popping out of a tea cup after Finn left the junkyard after getting bored of spying on the Ice King.

BMO Noire
Ronnie, the mouse, is a suspect in the theft of Finn's sock.

Storytelling
Squirrels appear in "Storytelling." They are sleeping in a hollow tree when Finn peeks in the hole to see if they are kissing. Finn's presence startles them so they run in a circle and bump heads, knocking each other out.

Notable rodents

 * Science
 * Ronnie
 * Squirrel (The Duke)

Appearance

 * Rats generally have grey or dark navy blue bodies with grey tails.
 * Mice have a similar appearance to rats, except that their ears are larger and they do not have protruding front-teeth.
 * Squirrels are brown to red-brown and have bushy tails, curled at the end.