Appearing in various areas of the Great Palace, they are similar monsters to Fokka. Unlike Fokka, they do not have arms, and instead has two large wings, although the creature does not fly. They also have a tail, and a feminine-looking chest. Their movement pattern is similar to a Guma: they hop up and down while moving back and forth, spitting fireballs in an upward arc. Unlike the Guma's chain hammers, these fireballs are a lot less predictable and will sit and burn on the ground for short periods of time, and even move along the ground towards Link when they land. Despite this, the method for dispatching these monsters is similar: Link must get underneath the arc and strike at the monster with his sword.
Fokka and Fokkeru are the first bird-like race shown in the Zelda series.
Given the Fokkeru's similar name and more feminine form, it is possible that the two monsters are intended to be male and female variants.
The name "Fokkeru" may have been inspired by the Dutch/German word "vogel", which means "bird". Specifically, the German pronunciation is what seems to be the basis.
The Futashiba Guide artwork (which was not created by Nintendo) appears to be a complete misinterpretation of the fire-spitting sprite, and rather than appearing as a bird it gives it a bizarre alien-like appearance.
Fokkeru spitting fire
Futashiba Guide artwork of a Fokkeru, which appears to be a misinterpretation of the monster's sprite