ad 1. Oh, but there actually
are black butterflies; see e.g.
here. No idea though whether any of these occurred in Britain in Tolkien's lifetime.
ad 2. Oh yes, the Green Squirrel, fiercest of all variants, feared by the forces of darkness. When Mirkwood was cleansed after the War of the Ring and became Eryn Lasgalen again, it repopulated its ancient haunts in the forest and drove the Black ones out, scattering them all over the rest of Middle-earth... or so the rumour goes
.