Parmandur
Book-Friend, he/him
What does the list look like if you limit it to the AD&D line Monster Manuals?Mind flayer: First appeared in The Strategic Review #1, then in Eldritch Wizardry, for 0e. Became a core monster with the 1e Monster Manual. Have been core ever since. (Except in Basic D&D, where they never appeared.)
Beholder: First appeared in Greyhawk for 0e. Became a core monster with the 1e Monster Manual, have been core ever since. (In Basic D&D, first appeared in the Companion Rules, then became a core monster in the Rules Cyclopedia.)