Nicodemus is a fearless sociopath with millenia of accumulated knowledge, a nearly flawless dueling record, and the support of Anduriel, who can suppress magic at will, gain situational awareness by eavesdropping nearly anywhere, perform shadow/perception trickery, and is otherwise reputed to be deadly. Further, he has some degree of immortality/unkillability from his noose, although it is also a weakness to him in close combat and seems to not help him against the Swords in particular.
Nicodemus is visibly taken aback the first time he meets Mouse. I believe this happens in Small Favor, and I recall his exacts words being "What is THAT?", along with a description of him visibly recoiling or otherwise becoming nervous. Anduriel may recoil as well. At the end of Skin Game, after Butters actualizes a faith light saber from the shattered Sword, Nicodemus finally flees outright when Mouse joins the fight.
So, why is Mouse so startling and threatening to Nicodemus?
My thoughts on this are:
- Mouse, although outside of Nicodemus' Christian mythology and power structure, is highly threatening because as a temple guard dog he functions like a mobile, combat ready version of a Church threshold. He might be able to strip or suppress Nicodemus' power simply by getting near him.
- Nicodemus may not know what Mouse is, even though he can sense power in him. This seems unlikely given that one of Nicodemus' recent opposites in the Knights of the Cross is a Japanese swordsman and that Nicodemus has at least two millenia on the Earth to encounter other mythologies and supernatural realms. I find it particularly interesting that not only is Nicodemus threatened by Mouse, he may be unsure of what Mouse is. He is certainly knowledgable yet unafraid of powerful extra-Christian beings like Mab.
- Mouse may actually be or house something more powerful than the standard Foo Dog demon. Or, in improvising to draw power from non-standard sources since he is paired with a wizard who has no thresholded home to protect, he may have become something different and more powerful. In this case, Nicodemus knows exactly what a Foo Dog is but is taken aback by a Foo Dog carrying around a level of power it should only have inside its temple/home threshold.
- Over the course of the series, we come to understand that Mouse has a human level of intelligence, and in Changes we learn that he can talk, although Harry only understands him when turned into one of Lea's hounds. I don't know much from the mythologies Butcher draws from in creating his in-universe Foo Dog, and Butcher has not explained all that much. Can we know if it is remarkable for a Foo Dog to have this level of intelligence?
- Foo Dogs are the scion of a temple dog and a celestial spirit. Any celestial spirit? Potentially a godlike/god-level spirit?
What other possibilities am I missing? What other interesting clues and tidbits tell us something important about Mouse? What is Mouse, if anything more than an innovative Foo Dog, and what might we expect from him going forward? Butcher likes to build up to interesting twists and convervenges, and it seems like Mouse is ripe for one of these.
Footnote: Unfortunate that there are not any tags for the religions/mythologies Butcher drew from for Mouse; Buddhism, Shinto, Tibetan, etc.