Final answer:
A boulder with a differing composition from the nearby rock that is found in an unlikely place is likely an erratic, usually transported by glacial action. If the rock is of extraterrestrial origin and has survived atmospheric entry, it is termed a meteorite. Identification might require laboratory analysis, especially after weathering.
Step-by-step explanation:
A boulder that is found in an unlikely place and/or has a different composition than the underlying or nearby rock is possibly an erratic. An erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. Erratics are often transported by glacial ice, often over distances of hundreds of kilometers.
When considering extraterrestrial rocks, any fragment of interplanetary debris that survives its journey through Earth's atmosphere and lands on the surface is called a meteorite. Meteorites can sometimes be mistaken for erratics due to their unusual appearance and differing composition from local geological materials.