So I applied for a job at a local Petco that's opening up. They sent me an email saying I meet the requirements and that I should come to an open house to talk to somebody about possibly working at the new store.

I've never been on a job interview, but I know that I would wear something nice. However, I have no idea what's gonna go on at the open house. Is it pretty much going to be an interview because I'm going to be talking to somebody, or is it just something I can wear something kind of nice but not too special to?
Smart-casual. Even if they don't officially interview you, the purpose of a recruitment day is to size you up.

Maybe email them back and ask what the structure of the day would be?
If you're going to work with animals, you have to become an animal.

Wear a horse costume.
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Collared shirt with slacks. Nothing any more fancy is necessary. Its probably gonna be an informal interview/tour. You should email or call them back and ask if you should bring anything and ask what the open house is going to include