In general, interesting and dangerous behaviors should be spread over more monsters rather than crowded onto a smaller set. For example, mind flayers have 3 unique things: a long-range psychic blast, a stat-reducing attack that causes instadeath (that uniquely ignores life saving) if it reduces the stat too much, and amnesia; any 1 of these things would make a monster species interesting by itself, but packing them all into mind flayers just makes them a genocide target.