Basics

Plastic filaments which flex upon application of force
Good for many unique uses such as joints
Can be problematic however as they can clog and jam extruders, and they print strangly (ie stringing etc)

Units of Meaure

Supposedly vulcanized can't melt
NEEDS MORE RESEARCH
A Whitepaper on The Melting Point of Natural Rubber
Probably wouldn't be an off the shelf option, but perhaps Rubber From Dandelions or something, although the complexity may be more trouble than it's worth
Also would most likely require vulcanization
May also require the addition of Carbon Black