Thanks @Phanjam ! It's working. 
Whew! Good to know

I was afraid my fix wouldnt work! Promise i will do proper testing in the future (i was just out of time earlier).
Two questions :
- With what kind of software you can edit mesh files ?
- What kind of information is stored in the mesh file ? Obviously the 3D model, but what else ?
I ask because I may have to use alternative mesh files for the pets I debug to can attack, like you did for the sand colossus.
Because the way I used to do that is to create an Attack1 animation base on another animation.
But I'm afraid it can alter the way the original monsters act in battle.
First, what did i do with the sand colossus?...
i did not do any mesh editing at all

all i did was make a copy of sand_construct.mesh and put it in the same RESOURCEDIRECTORY but renamed it to variantpet_sand_construct.mesh. So it is the exact same file as the original monster mesh except for the name. Then i just pointed to this new file-copy in the pet unit .DAT file ( <STRING>MESHFILE:variantpet_sand_construct ).
However there are 2 sister-files for
every .MESH - the .ANIMATION file (contains the definitions of all animations the MESH can do) and the .MATERIAL file (contains the definitions of what texture/image files to use on the MESH and how). All these 3 files must have the same filename, so...
1. variantpet_sand_construct.MESH
2. variantpet_sand_construct.ANIMATION
3. variantpet_sand_construct.MATERIAL
this is how they "recognize each other". Lastly these new files should all go in the original model directory
along with all the .SKELETON and image files, otherwise the .ANIMATION file (which references the .SKELETONs) and the .MATERIAL file (which references the images) will not be able to find those files. Of course you can make a totally new model directory and put all those files in there again, but that seems inefficient to me.
So now you have a completely different model you can use

where you can change (some) animations and even textures without affecting the units which use the original model (like the .ANIMATION edits i made which would affect even the regular Construct unit if i had used the same .MESH)
This should also address your last concern about making new animations without affecting the original units.

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Now about MESH files... really the only data in there is about the model (vertices, faces, UV maps and armature "bones"). Blender is a good tool to work with the OGRE .MESH 3d models used in Torchlight 1 and 2 - it's free, has a LOT of helpful communities for support
and is used by our 3D guru-members
@Vkoslak and
@Kva3imoda 
there are also
several good tutorials for it on our wiki.
But based on the above discussion, you should not have to go as far as model-editing just to have new animations
