There are two extremes here:
1) A bone golem is a pile of bones - in terms of creation technology, it's simply a heavier version of a skeleton; it's harder to break, but it's not very agile.
An interesting point is that in this case, the invisible connection between the bones is stronger than the bones themselves; otherwise, there's no point to them - they would fall apart from any blow.
If it's made, it should be modeled after the earth elemental from the fifth game.
2) A necro-dreadnought - a disgraced necrolite is turned into a golem; I imagine this as a seventh-level earth elemental with a skull instead of a head, and the body is supported by the magic of the imprisoned lich, but it must definitely be a high-level unit.