。
Beavers rarely do any harm to humans.
Literal
Beaver [topic-は] person [to-に] harm [obj-を] add [thing-こと] [topic-は] rarely not-exist.
危害を加える 'to do harm / inflict injury' is the stock collocation for 'cause bodily harm,' common in formal or legal registers. めったに + [negative] 'rarely' is a fixed frequency adverb frame requiring a negative predicate, in the same class as 決して, あまり, and the negative-only reading of なかなか. The structure nominalizes '(beavers do harm)' as こと, then evaluates it existentially ('that thing rarely exists') — a common nominalize-and-evaluate pattern.