Slim Jims are an industrial knock-off, created somewhere in the mid-70's for the more expensive line of air dried hunters sausages.
There are 2 main categories of "real" sausages. You mention Landjaegers - originated in the Swiss Alps, and intended to give instant energy to hunters as they traversed the Alps away from home and food sources for weeks. On this continent, there was pemmican. Native American made of game (typically deer or bison) and berries. Same idea. Instant nutrients that is shelf stable and can last virtually forever without refrigeration.
Pemmican is easy to make and there are numerous recipes you can find.

dod