1. So, what is this type all about?
The idea behind this type is to introduce an unconventional synthetic type for an RPG. Most RPGs, being of medieval origin, do not have rubber or plastic enemies. However, Pokémon is an urban fantasy franchise with sci-fi elements, so a Rubber-type is perfectly feasible... in theory.
2.Where does this name come from?
The name is a perennial joke type. First known instance comes from Serebiiforums' Joke types thread:
This type has expanded since the popularity of Deviantart and /vp/ into a serious suggestion. One suggestion by LeafyHart can be found here:
3. Does this type have an in-game source?
No. There are tons of elastic moves like Constrict, Wring Out etc., but this type is largely argued as a sub-group of a larger Synthetic-type that would include both cybernetic and plastic synthetics. Only species that are arguably composed of plastic are Trubbish and Drifloon, though this type is argued for all elastic species, including Buizel's collar.
4. What would be the ideas behind such a type?
Mainly synthetics composed of organic materials, such as tires, action figures, as well as general elasticity, plasticity and malleable species can be used.
5. What would be the type interactions of such a type?
Generally argued things are weakness to Fire and Ice, lots of defense and immunity to Electric. Anything else can be argued for anything.
6. What would be the distribution of such a type?
Probably similar to Rock. Most of Plastic-type species would probably be Mineral or Amorphous, so they would be easily accessible but few.
7. Future comments:
This type would be largely used as an earlier variant of Steel-type. It would presumably have access to Limber or a type based immunity to paralysis. However, it would probably used for mid-game species to make Joke Pokémon a bit more powerful. It would also be useful for making merchandise.