1.
ghost, monster, spook
A general term for ghosts, monsters, or supernatural creatures. Very commonly used with children and in casual speech. Can refer to traditional Japanese spirits or any scary supernatural being.
おばけ is a broad, casual term for any ghost, monster, or supernatural creature. It is the most common word used with children and in everyday speech.
Writing
Can be written as お化け or 御化け. Hiragana is standard, especially in children's contexts.
Forms
From 化ける (to transform, take another form), with the polite prefix お.