The Brown-tail Moth has a white topside and mainly white wing underside but as its name indicates, it has a dark brown, fluffy tail.
Wingspan 40mm; UK flight time Jun-Aug.
Groucho here (centre) has the big feathered antennae of males. The undignified last picture shows the brown tail.
The hairs of the black, hairy caterpillar are known for causing intense irritation and a rash on human skin.