How did the Rhine originally look like? (before human intervention)
It would help if you'd ask a more clear question -- it's a dynamic river system so it is literally changing its shape every second! The story over long time-scales can be found on Wikipedia: https://en.wikipedia.org/wiki/Rhine#Geologic_history . You can find some more detailed information with some overview maps (esp. Fig 3) here https://www.sedimentologie.uni-freiburg.de/staff/Preusser2008NJG.pdf .

In broad terms, the Rhine in its current state is due to two processes. First of all, the North Sea Basin, i.e., North-West Europe is sinking constantly, providing a great place for rivers to deposit sediments. This is why the Rhine ends up in the Netherlands, because the Rhine can deposit large amounts of sediment without silting up and blocking its own course. Second of all, the formation of the Alps caused a huge potential for sediment transport (realizing that all that rivers really do is transport water and sediment from high points to low points), and the Alpine orogeny furthermore caused the creation of the Rhine Graben. The Rhine thus found the perfect source (high Alpine mountains), channel (Rhine Graben) and deposition space (North Sea Basin).

The story over smaller time-scales also plays over smaller distances. The Rhine changed its course during glaciations for example, because the advancing ice caps pushed sand out in front of them (moraines), in the Netherlands this literally pushed the rivers into a more southern course. The story about these details goes on and on and on! If you have any specific questions, you may want to contact the Geological Survey of the Netherlands or Geological Survey of Germany, they have people who dedicate their lives to studying the past and the present Rhine!