Old (#15)
Yes, you have to replace a default item in order to view custom opacity on an item (Assuming your hero does not natively support it). In order to do so (Use Dota 2 Test for this so you don't screw up your main game) follow these steps:

1) Go into your main steam Library, right click Dota 2 Test, go to properties, updates tab, and from the drop down choose "Do not update this game automatically".

2) Using GcfScape, extract all of the game files from pak01_dir.vpk located under dota 2 test/dota into dota 2 test/dota. This essentially de-compresses all of the game files.

3) Rename all .vpk files located within dota 2 test/dota to something like pak01_000.vpkOLD. This will cause the game to load the uncompressed files rather than using the .vpk archives.

4) Using Studio Compiler, de-compile whichever default item you wish to replace in order to obtain it's .qc file. In this case it would be /dota/models/heroes/disruptor/mount.mdl. The only line that needs to be edited in the .qc file is the very first one; $cd "<insert your file path here>"

You may use any standard text editor for writing .qc; I prefer notepad++ as you can get a .qc language plugin for easier reading.

5) Export your custom model (as a .smd) from your 3D program being sure to follow the naming conventions of the default item you are replacing.

6) Using GuiStudioMDL, load up the relevant .qc file and compile your new .mdl; copy it into dota/models/heroes/<your hero here>.

7) Using VTFEdit, import all of your .tga textures into the program and simply export them out as .vtf files; follow the naming conventions of the default item you wish to replace.

8 ) Using a text editor, modify the .vmt file for the slot of your choosing and add $alphatest 1 somewhere in the middle of the code. I generally place it right before the Proxies line.

9) Launch Dota 2 Test and use the Loadouts window located under the Customize tab to view your custom item. It should show up as the new default. It should also work in-game as well if you create your own lobby, join a team on All-Pick, populate it with a passive bot or two, and choose your hero to view your item inside of the game map.

That being said, Spudnik may actually be correct about there being a rule against using opacity on heroes where it isn't already supported. Quoting the workshop page
Originally Posted by Valve
Alpha channel of 32 bit "_color" texture can be used as opacity for your item if alpha is allowed for this hero.
So keep in mind that you may not be able to use alpha cards for the spikes and may have to model them out. Lucky for you the mount has a rather high polygon budget