The commonly understood meanings are in a wild state and resembling a wild animal especially when referring to a domesticated animal having escaped from captivity or domestication, e.g. a feral cat. But now in Michigan, it apparently will mean any pig not raised on an industrial hog farm.
Small farmers, raising pigs of other breeds than those used on industrial hog farms, will become felons under the new law, passed in 2010 and scheduled to take effect April 1, 2012.
Here's a good article describing the situation in detail: http://www.alternet.org/story/154752/