In this article we are not going to look at Messianic Eschatology, but rather at a surprisingly wide range of rabbinic texts which do make reference to a concept of Armilus.
This is more than a mere translation. It's a clear insertion of a new word - 'Armilus'.
“Why is Armilus given permission to kill Mashiach ben Yosef? –To break (test) the will of the dissenters of Israel who have no faith, who will say; ‘this is the man we trusted in and now he is dead!’. They will have no hope left and will turn from the covenant of Israel and join the nations...”
“A king will arise whose name is Armilus and he will wage war against them and destroy the city (of Jerusalem) and plunder it, and this man from the Tribe of Yosef will be amongst those killed...”
 “In 1979, Avigdor Shinan...identified 122 expansions in Genesis that appeared in....the Palestinian (Yerushalmi) Targums and the Pseudo-Jonathan. He argued that these expansions represented the interpretations...of the translator, and could not be used to investigate the Targums’ theology.” See The Targums: A Critical Introduction By Paul V.M. Flesher, Bruce D. Chilton, p. 96 (Parenthesis mine).