In the icon files, there is an icon called "mexicanDuende.vimg". You can see the image if you change it to a.png file.  It appears to be a model of the allegedy alien remains shown off by the Mexican congress. 

Does anybody know if this is an actual item found in game?

Duende image in question:

there is many easter egg/sandbox items that are unobtainable normally, a few can be acquired by secrets with the funny setting turned on, but a majority are used only for the sandbox, and I believe there is also a set of items you can't get even in sandbox that the dev plays with for testing/their own fun