The “main man”-goose of this genus is the slender mongoose (Galerella sanguinea), which has a range going over most of sub-saharan Africa, except the really wet bits and the really dry bits. There are three related species with more restricted ranges: the cape grey mongoose (Galerella pulverulenta) of South Africa; the Angolan slender mongoose (Galerella flavescens) of Angola; and the Somalian slender mongoose (Galerella ochracea) of Somalia.
In general, these mongooses are of the solitary persuasion, unlike those meerkats and craziness. The most notable thing about this lot is that the slender mongoose is the most arboreal of mongooses – it is as adept at climbing trees (and hunting things in them) as a squirrel. One indication of this is that, like squirrels but unlike other mongooses, slender mongooses can run head-first down a tree trunk, facing downwards. Another indication is that, apparently, birds will often gang together to mob slender mongooses and drive them about, while ignoring other mongooses, less adept at climbing.
A couple of quick notes about mongooses in general. Despite being more closely related to cats than to dogs, they share dogs’ trait of non-retractable claws. And the pupils of their eyes are a weird oblong shape like those of goats. Just FYI.