Simplify the following text for a Grade 9 native English-speaking person.
Original: In Mexico , they are mostly found in the northern states , which are the southern end of the great plains : northeastern Sonora , north and northeastern Chihuahua , northern Coahuila , northern Nuevo LeÃ n , and northern Tamaulipas ; in the U.S. , they range primarily west of the Mississippi River , though they have also been introduced in a few eastern locales .
Simplified: In Mexico , prairie dogs are primarily found in the northern states which are the southern end of the great plains : northeastern Sonora , north and northeastern Chihuahua , northern Coahuila , northern Nuevo LeÃ n , and northern Tamaulipas . In the U.S. , they range primarily to the west of the Mississippi River , though they have also been introduced in a few eastern locales .