The term "beastie" has multiple meanings depending on its context.
1) In a general sense, it can refer to any small animal or insect such as a mouse, rat, spider, ant, etc.
2) It can also be used in the plural form "Beasties" to describe imaginary creatures that resemble animals but have human-like qualities, often found in children's literature and cartoons (e.g., Teenage Mutant Ninja Turtles).
3) In some regions of Scotland, it is used as a term of endearment for someone or something, similar to "love" or "dearie". For example: "Oh, ye wee beastie!" (meaning "Oh, you little creature!").