As predicates, (words used to modify the subject) both ある and いる are used to express the idea of "existence". ある is used for inanimate objects, (living or non-living) and concepts/abstractions. いる is used for MOVING living objects and possibly for moving but non-living objects such as vehicles.
One other distinction between these predicates is that ある can indicate existential possession while いる indicates existential location. This distinction, for example, allows the two phrases:
(1) 子供がある。meaning "I have children. / I have a child." (or he/she has / they have, if context provides a subject other than "I")
(2) 子供がいる。meaning "There are children (there). / There is a child
(as is noted in the comments on this thread: When is it okay to use あります with a living subject? this use of ある to indicate possession of a living thing is only acceptable in cases where the relationship is permanent, unchanging. This fact also explains the examples that Chocolate used in her comment below.)
In the example from your story, 者もあった it is possible to argue that 者 is an abstraction of "person/people" (for example, the way the words "those" and "some" can be used when speaking of people), and that this abstraction makes 者もあった acceptable... However the use of いる in its "location identifying" nature would seem to make 者もいた the more logical choice... so what is going on?
Unfortunately for learners of Japanese, the answer seems to be that this style of using ある in an existential way even for animate subjects, is a literary convention in Japanese fiction. As has been pointed out in one of the answers on the link above:
Furthermore, as is well known, even the predicate `ある` can be used with animate subjects.
(This COULD be an extension of the fact that fictional creations are abstracts... but that might be pushing the logic.)
In any case, your image that the narrator is slightly ridiculing those who suggested a fox hole, is pretty accurate. It's a very dismissive sentence. You are identifying そんな and も just fine.
Any translation has some wiggle room, always, so I feel that your translation is alright, though I might change "a person" to "some people" or "some":
"Could it be a fox hole?" / "I wonder if it's a fox hole."
There were even some who said things like that.