An echo mouse is a species of mouse-like beast demon inhabiting Bonesborough Library. It has the ability to consume books so it can display the contents of the pages at random. Amity explains to Luz they are found at the library due to their ability to preserve books that they have eaten.
Echo mice are small mammalian demons that resemble real mice and are roughly the size of Luz's hand. They are primarily beige in color with long tails that end in a quill-like tip and skeletal heads with two large buck teeth.
In "Through the Looking Glass Ruins", Luz and Amity discover a mouse eating the pages of Phillip Wittebane's diary when they had been looking for it. This frustrates Luz until Amity explains its abilities and the mouse shows Luz information.
In "Hunting Palismen", Luz wishes for the mouse to produce more pages of the diary before she goes to Hexside. When she returns home, she rants in front of it about how she wants her future in the Boiling Isles to go.
In "Knock, Knock, Knockin' on Hooty's Door", a diagram of the echo mouse can be found in the "beast demon" section of Hooty's bulletin board. By this point, the echo mouse is now out of Hooty's stomach and back in its enclosure, but it fails to give information to Luz. Later, the echo mouse is finally able to project new information to Luz and Amity.
In "Eclipse Lake", the echo mouse gives more information to Luz, Amity, Eda, King, Willow, and Gus, revealing that an ingredient needed for the portal door is Titan's Blood. When a sick Luz accidentally triggers the mouse by bumping its head, it projects a moment where Philip Wittebane describes his failure to get Titan's Blood, making her worry about the safety of Amity.
|The Owl House Main Theme:||Absent|
|The Owl House Credits Theme:||Absent|
|1. "Separate Tides":||Absent||11. "Follies at the Coven Day Parade":||Absent|
|2. "Escaping Expulsion":||Absent||12. "Elsewhere and Elsewhen":||Appears|
|3. "Echoes of the Past":||Absent||13. "Any Sport in a Storm":||Absent|
|4. "Keeping Up A-fear-ances":||Absent||14. "Reaching Out":||Absent|
|5. "Through the Looking Glass Ruins":||Debut||15. "Them's the Breaks, Kid":||Appears|
|6. "Hunting Palismen":||Appears||16. "Hollow Mind":||Absent|
|7. "Eda's Requiem":||Flashback||17. "Edge of the World":||Imagined|
|8. "Knock, Knock, Knockin' on Hooty's Door":||Appears||18. "Labyrinth Runners":||Absent|
|9. "Eclipse Lake":||Appears||19. "O Titan, Where Art Thou":||Absent|
|10. "Yesterday's Lie":||Absent||20. "Clouds on the Horizon":||Absent|
|21. "King's Tide":||Absent|
- The echo mouse is most likely inspired by the works of surrealist filmmaker Jan Švankmajer, who is famous for his live-action/stop-motion hybrid films which tended to include very off-putting creatures that were made from real carcasses and taxidermies.
- In "The Intruder", King explains that demons with black eyes are normally vulnerable to light. It is not known whether echo mice also have this weakness, but it seems likely due to the species' predilection for living in dark places, such as the Bonesborough Library's Forbidden Stacks.