As @Rizz Carlton previously pointed out, this individual is not the Javan subspecies usually seen in other zoos, but rather the mainland Southeast Asian subspecies (most likely Ratufa bicolor leucogenys) since it has a darker color compared to the lighter Javan subspecies
This is also the only one non-domesticated mammal in Aviary Park, while the rest of the park's mammals are domesticated animals (donkeys, goats, sheeps, horses, guinea pigs and rabbits)
@Fargusno this subspecies also exist in the Natuna Islands (though the exact subspecies is still contested). Aviary Park have a clear attention to smaller or under-represented Islands and species, so the Natunas is most likely their source of the giant squirrels (Whatever happened to the Natuna Islands exhibit).
They also still have meerkats, one species that is "un-domesticated".