A short quotation at the beginning of a chapter or article is called an epigraph. They are primarily ornamental and are not discussed subsequently in the text. The MLA Handbook does not discuss epigraphs, but the MLA Style Center site does give guidelines for the 8th edition:
The epigraph itself is not put in quotation marks. The epigraph source, which is sometimes preceded by a dash, includes the author’s name and often the title of the work, as in the example below. The source should not be included in the works-cited list unless you refer to it in your discussion. Bibliographic information is not required for an epigraph.
Here is an example of an epigraph:
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