Deposits and guarantees are normal in the market especially if you intend to rent an expensive apartment or house. The amount of deposits or guarantees has to be decided with the owner.
You should get advice from an independent third-party if you are unsure about a rental contract. Be aware that an agent may not represent your best interests (they get paid when you rent, so are motivated by closing the deal) and that you should really understand what you are committing to in order to avoid unpleasant surprises later.