unknown is not the name of the material. it has a name, the game just failed to pull it up and display it. Its a known bug in DF. names of most things come from the material type definition, so in this case you have a plant material in a liquid form on a creature. in that object ( the liquid plant material on the creature ) it has a pointer to its material. Now then that pointer is to an index in a table to locate the material data. DF basically uses the pointer to pull up its ID then passes that back to produce all sorts of things, such as names, and get its material information for all sorts of other things(weight, density, boiling points, etc,).
Now then the issue comes in when such a pointer grabs an ID beyond 32,xxx range. because when it goes to grab the name, its just indexing the material table and any index in DF beyond 32,xxx fails when passed to a table array (Its one of several things that are broken in DF). generally this isn't an issue, vanilla DF barely has 5k materials after all the metals, all the inorganics, all the creatures, and all the plants, along with all their separate part materials. Where does that leave us? well we have a pointer and that pointer is used to access the other parts it needs (its faster) so we know its something that has a deadly poison possibly on it if that's what killed the were.... but without accessing a DFHack and writing a script to search for the material's name through the pointer, we wont know what it is.
So yes DF failed to "find" the name, because its broken in certain ways, and those things get called unknown. It does not mean they don't have a name.
