I am currently developing a campaign setting using the rules and lore for Mouse Guard. The setting will be similar but not in the same geographical location (Northern Michigan) that the original was written in. To that end I will be bringing in additional animals and developing their stats.
In MG individual creatures come with Nature scores that determine the difficulty of the encounter. Additionally creatures might also have "equipment" in the form of their physical features (beak, claws, tail) that might add modifiers to the die rolls. The variation in these could greatly impact the difficulty of the encounter for the party of Mice.
I'm looking for expertise in balancing new creatures when introducing them into a Mouse Guard campaign.
I'm looking for answers using the "Good Subjective" standard and experience on how specifically balancing was done.