The word for monster in Italian is mostro (masculine, plural: mostri). Be careful not to confuse this word with the similar sounding mostra, the Italian word for exhibition!
As in English, the word mostro, in its most literal sense, refers to a large, ugly and frightening imaginary creature whose origins may be religious, literary, mythical or partially based on reality. Popular examples include the Loch Ness Monster (Mostro di Loch Ness) and Frankenstein’s monster (Il mostro di Frankenstein).
C’è un mostro sotto al mio letto!
There’s a monster under my bed!
Figuratively speaking, mostro may also refer to someone who is particularly ugly, often due to an unusual physical abnormality. Referring to character rather than appearance, it is also used to describe an unusually cruel person such as a criminal who commits unspeakable crimes.
Quel mostro merita di rimanere dietro le sbarre per il resto della sua vita.
That monster deserves to stay behind bars for the rest of his life.
By saying that someone is un mostro di (qualcosa), the implication is that they are a master in a particular field. For example, un mostro di bravura is a genius or prodigy whereas un mostro di cattiveria is a master of cruelty.
A related term is mostro sacro (lit: sacred monster), a person who has reached unsurpassed levels in a certain field, thus becoming an object of universal admiration. The expression derives from the title of a drama by French writer Jean Cocteau called Les monstres sacrés.
Hitchcock è un mostro sacro del cinema.
Hitchcock is a cinematic genius.
From mostro we also get the diminutive mostricino (little monster) and the derogatory mostriciattolo, which refers to an ugly or misshapen person or animal that inspires repulsion. The latter is also a funny pet name for a cheeky child.
Two other important words that are related to mostro are mostruoso (monstrous, horrific) and mostruosità (monstrosity).
Il Mostro by Roberto Benigni
Il Mostro is a comedy directed and interpreted by Roberto Benigni, who you might remember from the Oscar winning film La Vita è Bella (Life is Beautiful). Il Mostro tells the story of Loris (Benigni), an unemployed man who is mistaken for a serial killer nicknamed The Monster.
In the funny scene below, you see Loris’ clever plan to steal some goods from a supermarket.
Unfortunately I couldn’t find a clip with subtitles, but here are the dialogues from the second minute:
Signora con il passeggino (Woman with pram)
Oh, cosa succede?
Cassiera 1 (Cashier)
Scusi signora, venga, venga qua un attimo.
(Sorry madam, come, come here for a moment.)
Commessa (Shop assistant)
Scusate, permesso, scusate un attimo, permesso.
(Excuse me, sorry, one moment, excuse me.)
Uomo con la giacca beige (Man with beige coat)
Cassiera 2 (cashier)
Signore mi scusi, può venire qui un momento?
(Excuse me sir, can you come here a second?)
Commessa al telefono (Shop assistant on the phone)
Ma che succede? Qui suona tutto… Ah, ah ok va bene grazie, grazie.
Niente, niente, fate passare, è un guasto, è un guasto non si preoccupi.
(What’s happening? Here everything is going off… Ah, ah ok that’s fine thanks, thanks.
It’s nothing, nothing, let them through, it’s faulty, it’s faulty.)
Cassiera 1 (Cashier)
Vada, vada, è un guasto.
(You can go, it’s a malfunction.)