I presume that X terminals did not appear at the same time as X Window because Project Athena <https://en.wikipedia.org/wiki/Project_Athena>, which created X, had its users use "real" workstations from the start, the IBM RT PC being the first. I don't know if MIT ever deployed any X terminals but, as I understand it, one of the tenets of Athena is that every workstation is a full-fledged remote login-capable node of the Athena cluster.