If you hit a deer while driving your vehicle this would normally fall under the comprehensive portion of your auto insurance policy.
Typically any animal (be it a deer, dog, bird, etc) that you hit with your vehicle or that hits your vehicle is covered under your comprehensive coverage. Comprehensive coverage, which is also defined as "anything" Other Than Collision Coverage (OTC) normally covers your vehicle, and sometimes other vehicles you may be driving for losses resulting from incidents other than collision.
For example, comprehensive insurance covers damage to your car if it is stolen; or damaged by flood, fire, or animals. This physical damage coverage pays to fix your vehicle less the deductible you choose.
To keep your premiums low, select as high a deductible as you feel comfortable paying out of pocket. Coverage is limited to the terms and conditions contained in the policy so if you have a specific accident or situation that you want to know if your insurer would consider to be collision or comprehensive read through your policy and speak with your insurance agent.