You can use pretty much anything as a skeleton.
For instance, if you take a Null, Cube, sphere etc. you could put it on the leg of the croc. Simply parent the object to a bone (bone is parent). You will be prompted to make it part of the skeletong Y/N. Say yes, and you can deform the envelope with the object, just like a bone. Tag the points you want controlled by the object. Use Skin/Env Assignment/Assign Manually and select the object. Now they controll those tagged points.
Once you have it set up. If you for instance, scale the object, the envelope will appear to buldge in that area. You can use expressions to make the object scale automatically when you move the bone etc.
Note: You may want to use a null parent for the object. Then once you have the object positioned use effect/freeze/rotation and scale. Then when the null parent and object become part of the skeleton, you can still see scale of 1 and rot of 0 on you object. Not sure if this makes since, but try it and and let me know if you have problems.
Urp