What does a black weight icon mean?

[OmniLordAlpha]

The question's basically in the title there. I've seen this on a T-Rex in single player and an argy while playing on a server. When I press H no additional info is displayed to explain it. There -were- mods involved, so perhaps it's from one of them, but I was wondering if anyone else has seen this. Could you please explain it if you have? Thanks in advance.

[The Waffle that Lurks]

The black weight icon means that inventory is full. Drop items to remove it.