Aquamonster

Aquamonsters were fish like creatures native to the Aqua planet. They could only live in the planet's oceanpheric currents.

When the First Doctor, Ian Chesterton and Barbara Wright, Ian noted that he saw a fish fly. In reality, there were patches of invisible liquid in which the fish swim. However, the oceanpheric current ended up catching up to the trio in which an aquamonster attacked Barbara. As they fell out of oceansphere the aquamonster had no choice but to let her go for it could not live outside of its liquid patch. (COMIC: Dr. Who on the Aqua Planet)