"It would be very easy for me to stop him, if you decide that's what ought to be done."

Does the bestiary have such a thing as an index? Any way to easily sort entries by reference to fire?

There is an index, but it doesn't help much with 'fire'. The entries are grouped by region of the world.

There are helpful annotations by someone, though, on the table of contents, circling the relevant entries. 

What entries have been deemed relevant?

Cherufe, dragon, firebird, hellhound, ifrit, and phoenix have all been identified as possibly relevant!

Alphabetical order, then. What do these entries have to say for themselves?

The cherufe is a creature made of lava; none have actually been encountered in North America, according to the helpful annotations. 

Dragons may not actually be able to shapeshift; unlikely to pose as a deputy. 

Firebirds have been seen taking human form, and their tendency to burst into flame is well-attested. No history of moving corpses. 

Ifrit do not typically resist flame, so lighting themselves on fire wouldn't do much good.

Hellhounds are typically wreached in flame to collect the fallen; fits the fairy's observations, but they typically don't stick around.

Phoenixes, like firebirds and hellhounds, can have a flame aura, and are tied with rebirth. The annotation here reads "Kitsune?"

Where does that cross-reference lead her?

...nowhere, since the annotations are a later contribution, by someone with access to the file, and the original author evidently didn't consider "Kitsune?" to exist. 

The others are talking quietly in the examination room. 

Hmm. Inconveniently unstandardized.

Probably hellhound or phoenix of those options. Rebirth doesn't really seem indicated, so hellhound. What other information is there?

The hellhound appears as a spectral creature of flame. Shapeshifters that take the shape of a dog (see also: werewolves, werecoyotes) typically attack vulnerable humans. Hellhounds are less violent than the others, but frequently pick off the weakest and sickest members of the population. If one looks at you three times, your death is sure to follow.

Werewolves and werecoyotes?

Werewolves! Reasonably common canine variety of shapeshifter, known for their aggression, violent impulses, claws, fangs, and pack structures. Frequently pose as humans, live on the outskirts of human civilization but participate in it to varying degrees. Any human can be turned by a bite, though it can also be fatal, but only from an alpha. 

Werecoyotes are solitary, killing their young and having no packs, but otherwise essentially similar. 

The easiest way to force a werewolf to reveal itself is to produce a high-pitched noise. 

"It would be nice if there was a source not written by people who spend their lives trying to kill these things. Is there one?"

"The druids didn't really write anything down, except about their traditions. We can take you to meet Dr. Deaton, a druid friend of the pack."

Sigh. "What's the point of integrating with a culture that has invented writing if you don't want to use that technology to record your useful information?" This question is not directed at anyone in particular. Nor is it said in a tone of voice that indicates that it is in fact a question.

Lydia snorts. 

"Druids have a bad habit of going for mysterious and objective and in tune with nature, over actually helpful."

"Druids pass things down directly, they don't really want this stuff to get out." 

"Hm. How inconsiderate."

"No one really does. Hunters, druids, werewolves, kitsune, wendigos...even the regular people who know don't tell."

"I suppose that must at least raise the possibility that there is a good reason for it."

"Not really. They just all like killing each other." 

"I think it's still dangerous to tell people without vetting them. There are a lot of ways it goes wrong." 

He looks at Ellie. 

"It's late, so Dr. Deaton won't be up right now. What do you need, sleeping arrangements?" 

"A cup of coffee in the morning will work just as well. I want to continue examining this bestiary."

"Lydia, can you host her for tonight?"