A ghast is a faerie creature who has the power to enhance the power of other magical beings. They are very like genies and are often employed by Genies as helpers.
It is likely that Oberon created Ghasts by magically altering a number of genies, allowing them to use their powers in tandem.
The average ghast has power meant for a specific task, such as transportation, conjuring, transformation, defense and so on. A powerful genie, such as the genie of Aladdin's lamp, may have up to a thousand ghasts working for him. The most powerful ghasts have an array of powers with which he can charge a magical partner.
Ghasts are rarely seen by the public, and so tend to ignore manners and appearance. They tend to be gaudy, slovenly, obnoxious and offensive. In other words, ghastly.