I am having difficulty finding out which national parks in Nepal do not legally require you to have a guide, using my phone and local data. There doesn't seem to be a single website, even a Nepali governmental one, to summarise that information. It's easy enough to find out how much entry fees are, but for guide requirements, it seems to be different in every website.
National parks in Nepal do not legally require you to have a guide. Guidelines, Rules, Acts, Policy, Regulation Collection 2073Language: Nepali Government of Nepal Department of National Parks and Wildlife Conservation, doesn't state anything that make guide compulsory for a tourist.
Two things that I have found concerning this question is: (National parks and Wildlife Reserves only)
My translation: Without entry card provided by authenticated person of the Government of Nepal no person can enter the National park or Hunting Reserves. (other part is exception for Government person)
My translation: Entering in National park or Wildlife Reserves will be your sole responsibility. If some accidents happen inside National Park or Wildlife Reserves Government of Nepal is not responsible for compensation.
SO, legally you are not required to have guide for National Parks or even wildlife reserve in Nepal but if you want one that guide must have license provided by Government.