Plankton

 is a single-cell creature drifting on the water that is a food for fishes. Even though they seem insignificantly small, be careful not to step on them. Plankton are light and can drift on the water.

Trivia

 * Plankton are organisms that live in the water column. This monster is one of the few in Ragnarok Online that actually matches a real-life incarnation in both name and form.