Pretendian is a slur used to describe Native americans in a derogatory way. Native Americans who claim to be pure, but are just mixed-breeds. I suppose this could refer to any mixed -ian race.
The word Pretendian is a racial slur that has been used historically to degrade and dehumanize native americans. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to native americans. If you are unsure about whether or not it is appropriate to use the word Pretendian, it is best to err on the side of caution and avoid using it in any situation.