Excelsior

Is it a waste to use these on ru's? Is there a chance to get anything beyond horned/barbed (other than any eles i get of course) ?

I only use them on ancient dragons, 3 level of destard! I think its the best for getting high level leather! And dont forget to skin the elementals!

But i might be wrong! Work well for me at least

The garg knife will randomly increase the type leather you receive for skinning, but only a certain number of levels -

Creatures the normally drop:
Barbed - Polar through Ethy
Horned - Barbed though Shadow
Spined - Only barbed or horned (I think)
Leather - Only spined or horned (I think)

From the list you can see that it is best to use a garg knife on creatures that will normally drop barbed, this includes dragons and ancient wyrms, which give 20 and 40 respectively. If you are after lower types of special leather (the polar through shadow), it is worth skinning drakes and wyvern, which both drop 20 leather. I never use a garg knife on creatures that drop less than 20 leather, like the kitsune. It is also not worth skinning anything that normally drops spined or plain leather, as it is too easy to get those leather types from just skinning the regular creatures that drop those types.

An extra note, when using a garg knife in Fel, you will not get double the leather drop. I don't know if this is on purpose or a bug.

When an elemental shows up, make sure to skin it as well. Using the garg knife on the elemental will randomly up the type of leather it drops. Another good option when skinning leather elementals is to use a masters knife to increase the amount of leather you will get; this will make the ele drop 60 leather instead of 50. On Frost and Ethy elementals, this is well worth the price of a masters knife.