This idea is based on the 2ed supplement The Tempest, where an ancient Diedne magus can control great beasts through their essence trapped in paintings.
The troupe found a dozen urns with these paintings, a dragon, a basilisk, a bear, a giant snake, ... and a squirrel.
After a lot of discussion, the magi decided to destroy all the urns, but now they can't stop talking about the dreaded squirrel. What if there are more?
And the poor squirrel never even got the chance to make its appearance.
The squirrel might have low stats, but has a lot of magic powers, able to control the forest in ways only the most powerful Herbam mages can imagine, having most Cr/In/Re powers (much like the water faerie queen has all Aq spells and the Holzfrau many He powers).
Maybe have the innate power to cast up to once per season "calling the council of the Trees" (ArM p139), and while the squirrel could be crushed with one axe blow, it doesn't matter because the squirrel is on the other side of the forest controlling trees with a variant of lord of the trees to attack and/or confuse any one coming into its forest uninvited.
Is that a squirrel with a human heartbeast?
There is always Ratatoskr, the secret stealer! The norse squirrel is a great gossip, so who knows ahat dirty secrets about the magi he has squirreled away? I like the idea that the memories are trapped like hologram recordings encased in nuts that Ratatoskr has hidden away for later. Maybe he is harvesting memories of the mundane world to keep him through the lean period after ragnarok where there isn;t much going on in the world anymore.
Anyway having a mythological squirrel that can scry on anyone anywhere, might be scary. Especially if their mischevious nature tries to blame the player charcters for the scrying.
Bob
True, true.
What fascinates me is that it does not matter the slightest what that squirrel can do. The characters do not have the foggiest idea, but the squirrel, in the right context, still stands as a mysterious threat.
This may not fully fit in Mythic Europe, but if you're not opposed to drawing inspirations from other RPGs, Rolemaster has Carnivorous Flying Squirrels, which can glide from trees and operate in piranha-like swarms of up to 500. 
God knows how that awful Diedne achieved such a Transformation on this pack of squirrels of virtue for them to want to eat flesh...
Nah ... I'd rather go for Monty Pytonesque killer rabbits.
As you said in game “I wish I had added a rabbit.”
Iron Druid- the squirrel conspiracy. After all squirrels are small, sneaky, have hands, and thousands of friends...
I'd say...
