There is no official explanation to that. Thus, leaving a plot hole into the story - but it can be very logically assumed they were rouge lions taken in by Scar at some point after Simba ran away. That after all is the only logic explanation seeing to how they evidently were not there in Simba's childhood.

Why they weren't in the final battle against Scar even though they evidently had been part of the pride, is another plot hole unexplained by official sources. This too can be very logically assumed to be about how Nala just didn't gather the entire pride when Simba told her to gather the lionessess but she left Zira & co. out and they were introduced to Simba at a later point.

But yeah, the point is there is no official information and it's left up to people's common sense + fan theories.