Protozoa

Etymology
Protozoa:

proto: "first" (from greek πρῶτος (protos), meaning "first")

zoa: "animals" (from greek ζῷα (zoa), meaning "animals")

Explanation
Protozoa are single-celled eukaryotic organisms, that were presumably among the first (proto) organisms to develop which demonstrated animal-like (zoa) behaviours, such as being capable of movement.