In my view, an input device can have different features or aspects of its design that makes it ergonomic. Some features are more important than others. Some ergonomic devices are better than others in preventing injury. Others can be difficult to use and rather intended for people who are already injured.
Marketing has a different goal though: the one of chasing features to put on the info sheet to make it sell more. Sometimes a mouse is marketed as "ergonomic" pretty much only because it is shaped for the right hand ... but that does not necessarily make it more ergonomic if the shaped surfaces are not significant in directing the user towards a grip that leads to better hand/wrist health in the long run.
And sometimes a mouse or keyboard is called "ergonomic" for no actual reason at all than to make the cheap crap sell.
In my opinion, the standard mouse should be semi-vertical, slanting at 33-45° or so to provide a more natural angle to the wrist, but I don't think that it has to be
long in the hand so that you would have to move the whole arm to move the mouse.
I have posted many times that I use the WowPen Joy: it is light and semi-vertical but not long in the hand: it feels like using any "normal" mouse. It
looks weird though.
I think that the Microsoft Sculpt Ergonomic Mouse has a good shape too: it does not look "ergonomic" but it encourages the same slanting of the wrist. Unfortunately, because it is wireless with a battery it is a bit too heavy IMHO.
... mice that are marketed to be ergonomic ... sometimes have a trackball inside.
I suspect that what you are referring to are
only trackballs, which are supposed to stay put on the desk ... but
called "mice" because they have the same use. I've seen that many times in product listings. (I suppose normies could get confused when you say "pointing device". :-þ )
Trackballs are generally considered more ergonomic than normal (horizontal) mice because they either require a good angle to the wrist, or they don't impose any angle to the wrist at all and thus allowing the user to position the hand in any way that is comfortable.
