What figure of speech is represented in the sentence “He was sweating like a racehorse.”?

Simile is an explicit comparison between two different things. Usually 'as' and 'like' are used in Simile.
The bride hath paced into the hall,
Red as a rose is she.'
It is an example of a Simile. It has taken from Coleridge's Ancient Mariner.

Metaphor is an implicit comparison between two different things. A metaphor is a figure of speech that is used to make a comparison between two things that aren't alike but do have something in common.
Ex: Her tears were a river flowing down her cheeks.

Allegory: (রূপক বর্ণনা; প্রতীকাশ্রয়ী কাহিনী)
A literary form in which one story is told in the guise of another story.
Ex: Bunyan's Pilgrim's Progress

Litotes: Ironic understatement in which an affirmative is expressed by the negative of its contrary (e.g. I shan't be sorry for I shall be glad ).

Source: An ABC of English Literature, Dr. M Mofizar Rahman and Oxford.