奇怪! Is it called "last month's rent deposit"?

You can read the related Tenancy Act. The clause below is excerpted from the Ontario Tenancy Act for your reference. I believe you can review the similiar Act for your region.

"The rent deposit must be used for the rent for the last month before the tenancy ends. It is a violation of the law to use it for anything else, such as to pay for damages."