Shapechanging monsters are cognizant of when they’ve turned into a form that’s much stronger than their usual; if they happen to have an amulet of unchanging, they will wear it when in such a form.

Monster which is a shapeshifter, but regardless of what form it takes it always has an engulf attack. This is because it’s actually a variety of mimic in its normal form.

You can tell shapechanger pets to change shape (but not what to turn into) via chatting to them.

Putting on a ring of protection from shape changers and forcing a shapechanged monster somewhere on the level to revert to its normal form wakes it up if it’s asleep.

A b-class monster which is a shapechanger and may transform into any P, b or j monster.

When tame shapechangers polymorph, they attack creatures based on their current monster level (or perhaps the maximum of their current level and their new form’s level), not blindly use the new form’s level.