Hallucigenia

By far one of the strangest prehistoric creatures on this list is Hallucigenia. At max 1.38 inches (3.5 cm) long, this creature was originally constructed upside down and with its back and front reversed when scientists first created models. An ancestor to either arthropods or worms, Hallucigenia had 7 or 8 pairs of legs with rigid spines correspondingly coming out the top of its body.