Star Trek: 10 MORE Alien Encounters Before First Contact

That we know of, Pelia is the only on-screen Lanthanite we have seen. According to Spock, her people lived among humans for thousands of years before ever coming out, with their physiology being virtually identical to that of a human. 

Pelia boasted claims that she had lived on Earth around the time of Pythagoras, amassing a collection of historical artifacts that may or may not have fallen into her hands legally. In the 21st Century, she was based in Vermont and was thankfully able to help a displaced La'an get where she needed to go.

Lanthanites, possessing the lifespans that they do, were said to succumb to boats of utter boredom. They quite literally were able to see and do everything, when enough time had passed, so keeping things interesting was as important as breathing for them. This inspired Pelia to join Spock's sojourn (i.e. stealing the Enterprise), subsequently joining the ship's crew - for something to do.