I'm in accounting so I can tell youa bit about that aspect. The IRS doesn't consider it a business unless you've mad a profit 3 of the last 5 years and they consider intent. You have to have set out to make a profit. They, also, don't allow claiming most expenses from a hobby. When considering whether you made a profit, you have to consider everything; Food cost of equipment, cost of animals, cost of supplies, electricity, percentage of mortgage/rent goes for the space you use, etc. When you coonsider it all, it's pretty tough to make profit on your hobby. They would probably lose more money in deductions then they'd ever collect in missed tax revenue.
As was said there are individual rules with states, counties, and municpalities. I would check on-line if I were you or if you call do it anonymously. With current attitudes toward our hobby, you don't want to make yourself known if you can help it. They should have the info on their website. Even my bankrupt city of Flint, Michigan has put all city and tax ordinances on their site.