There are many good reasons to think that the “beast” in the book of Revelation refers to a person and not a mere symbol (contra idealism and historicism).
One of these reasons is found in how Greek grammar works. In Greek, a relative pronoun will agree with its antecedent in gender and number. But its case is determined by its function in it own clause.
There are exceptions to this grammatical rule. One of these exceptions is called gender deviation where the pronoun is not in agreement with the gender of its antecedent. David Matthewson in his Intermediate Greek Grammar states that this is explained by the sense of the antecedent and not its grammar (p. 49).
A good example of this is Revelation 13:14b: ποιῆσαι εἰκόνα τῷ θηρίῳ, ὃς ἔχει τὴν πληγὴν τῆς μαχαίρης καὶ ἔζησεν (To make an image to the beast, who has the blow from the sword and yet lives.)
The term θηρίῳ (beast) is neuter but the relative pronoun ὃς (who) is masculine. Using the masculine usage and not the neuter, what would be grammatically expected, suggests that the metaphorical term “beast” refers to a person. In other words, John envisions the beast to embody a personal (male) figure as an eschatological antagonist (Another similar example of the sense overiding gender agreement is John 6:9).
See Matthewson, pages 49–51 for more discussion on this point.