“孙行者” is 孙悟空's real name. He made up "者行孙” and "行者孙” himself. 孙行者、者行孙 and 行者孙 are all names.
"孙行者" escaped from Qwedcxza's(the name of the monster) cltuth -- the bottle gourd. He thought if he used a fake name like "者姓孙", he wouldn't be sucked into the bottle gourd when he responded. It turned out that the bottle gourd would suck in anyone as long as he responds. It doesn't matter whether he uses a fake name or not.
者行孙 and 行者孙 claimed that they were 孙悟空's younger brother. In fact, they were the same person, because 孙悟空 wanted to trick the monsters.
Hope it helps! :)