loan shark 
noun informal, derogatory
A moneylender who charges extremely high rates of interest, typically under illegal conditions: reports of exploitation and deceptive trading practices by loan sharks...
- Others make frequent trips to pawn off paychecks at an interest rate a loan shark salivates over.
- The rates may not seem onerous to borrowers when their only other source of credit is a loan shark.
- Credit card holders know that borrowing on their card without repaying in full at the end of each month is the most expensive form of borrowing available - short of approaching a loan shark.