I do not use it extensively, but I view it as a companion to Insectrist or Scientist. I actually (in recent years) rename it as Naturalist (also pulling Herbalist out from Scientist as a separate skill), and encourage players to think of it having similar factors as Insectrist or Scientist. So, that expands the skill factors a bit, first using the existing description of speak with beast and understand beast, but also allowing a mouse to act as a sort-of wrangler or handler getting the beast to respond to instruction (like Insectrist) or making use of animal byproducts (like Scientist). This necessitates a bit of added overhead on the factors.
Regarding the sentiment of low fantasy or high fantasy, I keep things much closer to the low-fantasy narrative: beasts can be spoken to and understood, but they have much less expressive language and little sense of grammar or vocabulary; beasts can be tamed and trained, but there will be an exchange of sorts–a cost of doing business with an animal–that comes as an added burden.
In this sense, having the skill itself allows casual exchanges of conversation, but a test would be toward establishing the rapport to negotiate a partnership or alliance. So, in those cases, the Loremouse skill is tested, then becomes a Helper skill from that point forward talking to a beast. I diverge from the rules by testing Loremouse Vs [animal] Nature rather than as a factored test; this means each individual animal might roll out differently when a loremouse is attempting to establish that opening rapport. Once established, then Haggler, Persuader, Manipulator, maybe Orator, and other interactions like Healer or Instructor, can take place so long as the Loremouse skill is offered as a Helper skill or the loremouse himself/herself is making that follow-on test.
Also, I diverge from the rules describing a versus test for understanding the Nature descriptors. I have a bit different list of factors, but I use a factored test against an Ob to determine if a character can recall or sort out the descriptors; I treat it a bit more similar to Hunter in that sense, and I allow Hunter to discover the descriptors, so a character could learn the same info without Loremouse rated.
In the sense of reducing the tests, take up a Wise! So, Hare-wise, Coyote-wise, Cardinal-wise, Songbird-wise, Waterfowl-wise, Serpent-wise, and many others I would allow as a stand-in for testing Loremouse when interacting directly with that sort of animal. So, even if a character lacks the Loremouse skill, they could take up a Wise that allows that aspect of communication or understanding. But, it is restricted to only that stated animal or animal type (and I don’t mind players selecting the specific animal, but I’d always encourage selecting the sort of animal rather than the specific animal, e.g. Predator-, Raptor-, Serpent-, Scavenger-, Forager-, Grazer-, Shorebird-, Songbird-, Amphibian-, or Critter-wise are much more often encouraged rather than Fox-wise, Hawk-wise, Milk Snake-wise, Skunk-wise, Raccoon-wise, Deer-wise, Gull-wise, Cardinal-wise, Frog-wise, or Cricket-wise. But, players pick what they want.
Now, the effectiveness of having Loremouse is slightly undermined by the idea that any mouse can learn to be an animal handler or wrangler; I’d describe Laborer as the lowest means of wrangling or handling an animal, even training. But this drives the Loremouse into a slightly superstitious realm that we might describe as a horse-whisperer or dog-whisperer–it’s someone with an uncanny sense of communication with those animals that goes far beyond the outcomes for a wrangler or handler. Maybe you have seen a docu-drama bio-pic about Temple Grandin; I’d say that’s an example of how much gap exists between a cowherd under Laborer tests, and Temple Grandin’s cow-whisperer expertise under Loremouse tests. It is a bit rough example to use, and you’ll probably want to watch the docu-drama to envision what I mean.
But, I did want to move Loremouse (as Naturalist) alongside Insectrist and Scientist (and Herbalist), to give a strong impression of this as a remarkable field of research-based study, and away from the meager keeping of pets; keeping a pet can be described with Laborer, becoming a biologist/zoologist can be described as Loremouse/Naturalist.
As for making the relationship, there are some insights from the box set supplement which includes rules for mounts. The rules of capturing and taming a mount can be insightful for building the process of shaping the relationship. It describes a Taming Conflict, which I totally endorse, but I would also allow for little bits of Loremouse and/or Instructor to blend in a training scenario after the Taming Conflict has been conducted to describe how much that tameness has seeped in or how much resentment remains against the mouse. Possibly, a game master could shape a Training Conflict using Loremouse and Instructor, and maybe Hunter or Laborer. But, I would more likely consider a series of tests of Loremouse, Instructor, Hunter, and Laborer (and maybe Healer) to describe the process of training over periods of time.
If I were really spit-balling new ideas, I might create a Training Conflict as a special means of adding, removing, or changing a Nature descriptor while using simple or complex tests of Loremouse, Instructor, Laborer, Hunter, Healer (and possibly Carpenter, Stonemason, Weaver, and Armorer) for common practice, keeping, and care of the animal.
And, all of that above must be informed by how much staging you want to have for players to manage. If you want the animal mount to be a temporary and fleeting cameo, just pull together a few simple tests; if you want the animal mount to be a frequent character in the campaign, use more complex tests and rely on exchanges between mouse and animal as a burden for continued partnership. If you want to focus the entire campaign on the relationship between cloakmice and mounts, that’s a candidate for conflicts and complex tests to capture, tame, train, practice, keep, and care for the animals.