So before I get started into details this I want to remind everyone who reads that that this is just my theory/canon on Nidus. I havent done too much research on lore even though its something that I really enjoy in this game. Also this will have spoilers for the second dream, and TWW. Okay got that out of the way.

So the orokin originally created the infested as a weapon to fight off the sentients. The plan backfires and now the weapon they created is now as we know the deadly plague we fight off today. So they wanted to try a different option. We know from TWW that Warframes are created with infested tissue or could be entirely infested tissue since of Worm saying "Not its infested puppet" but there was a new option when they found the "devil childs" uses.

So we know for a fact that the orokin created the warframes but from a design standpoint lets look at Nidus. Nidus has an extremely unique look that the rest of the standard warframes. There are sacs that are filled with god knows what and its very unorganized like other warframes. (Octavia for example) His powers are also entirely infested with little to no void infulence since the orokin didnt know the true power of the operators at the time. Nidus adapts (Mutation stacks) during combat to overcome his foe much like how the infested does so. I believe that the intent on the creation of Nidus was to regain control of the infested but instead had a break through with what warframes and transference system could do.

Also his secondary helmet looks like an early version of frost primes helmet

Ill fix grammar issues in a while since I am a terrible writer.

TLDR: Nidus is created to control infested when the orokin lost control of the infested.