Letchworth Garden City is home to one of the UK's largest colonies of black squirrels, thought by some to be a genetic mutation of the common North American Grey squirrel, but in fact a rare but not unique example of Melanism.
There are also muntjac deer living principally on Norton Common, but also increasingly elsewhere in the town (Jackman's Estate, for example where they often leave evidence of their presence on the allotments, much to the annoyance of allotment holders!).