I signed a lease which stated that there would be a $300 pet deposit ($200 refundable, $100 non-refundable), which isn't a problem, but now a few days later I get a call that I need to come in and sign a separate addendum that says I must pay an additional $200 non-refundable pet fee before I move in. Since I had already signed the lease days before I was told about this addendum, do I need to sign it/pay the additional $200?

Thanks!

You have a binding contract (lease). It's too late for the landlord to add clauses.

I am assuming you both signed the contract/lease, which makes it binding on both of you.

Well I'm doing this through a local apartment locator service so I'm not sure if the real owner has actually signed it yet, but I definitely have.

I'm not sure if it was the owner trying to add something else last minute or if the locator service just forgot to give it to me at the time, so if it was an error made by the locator service and I was actually supposed to sign the addendum at the same time as my lease, would the lost $200 then be between the locator service and real owner?

I just hate to start a new lease on bad terms with the owner if it's the locator services mistake, but as a broke college student I don't have an extra $200 laying around to just let it go.

You don't have to pay, and if you gave anyone any money, they have to give it all back.
Technically you can hold everyone to the lease as it was signed, but the reality is that it's not worth suing over.
It might be a mistake, but some of those locator companies are sleezy and want extra fees for themselves.

I'm also thinking that the locator company is trying to squeeze more money out of you to line their pocket. Sometimes they can be pretty sleazy and get away with it.

Yes, there was a offer and acceptance in writing with payment made, They can not change the terms of the contract ( lease) without your permission.