Horsefly bites look like any other insect bite, but may be larger than a mosquito bite. They can cause redness and swelling around the area of the bite, which is usually accompanied by an itchy feeling. In some cases, blisters may form at the site of the horsefly bite or even a small sore that looks similar to a pimple. In more severe cases, people can experience hives or rashes as well as fever and fatigue due to an allergic reaction to the toxin injected into their skin when they were bitten by a horsefly.