They are certainly up there. Non-humanoid. Not even carbon-based (emphasizing their alienness and the difficulty in recognizing their eggs as eggs
). Unusual life-cycle of only one mother (again emphasizing their alienness and the difficulty of noticing them). And yet there is a way to find common ground. They are not completely unlike us, because they have a generational reproductive cycle and thereby instincts similar to ours that we can relate to and understand. The episode itself is very fine and suffers mainly from some cheesiness in the monster attacks ("you go over here by yourself and get dramatically killed by the monster").
In addition to the Horta, I'll nominate the Vulcans, the cosmic cloud from "One of Our Planets Is Missing", the Q, the Borg, the Companion, the Tholians, the two-dimensional creatures flying into the cosmic string, the Tamarians, the Trill.