What is Kitbashing?
Kitbashing is a technique where you combine different model kits into one new model; like a collage.
They used that technique in the first Starwars movies. To make the large spaceships the film crew would buy model kits of WW2 tanks and stick the turrets to the surfaces of the spaceships to make them look more interesting.
With 3D models you can do the same thing. Just download a bunch of free models ( I wrote a Blogpost about that)and bash them together in an application like Zbrush. Use the dynamesh function a few times to merge all the parts together and then export the 3D model and get it 3D printed.