Octrolls are creatures native to SR388 resembling octopuses, found exclusively in Phase 5.
An Octroll's size makes it nearly as large as Samus Aran. They are normally calm, but will attack intruders who enter their territory. These cephalopods' attack pattern is nearly identical to that of the Chute Leech, though the former can jump much higher and cause more damage. Two of their tentacles possess fins, possibly used to float through the air. When destroyed, they nearly always yield large energy units.
The game's commercial depicts an Octroll rising from a blue liquid substance. In-game they thrive in the dangerous liquid. Since Metroid: Samus Returns depicts the liquid as purple, it is possible that the commercial intended to accurately depict the dangerous liquid, rather than water or some other substance.
Metroid II manual
Enemies (p. 23)
"When Samus comes close, they fall fluttering downward."