Lanthanum was the third rare earth that Mosander discovered, after yttrium and cerium.
After this, Mosander discovered terbium and erbium.
“Earth” is a term for a metallic oxide.
They are called “rare” earths not because they are rare,
but in the archaic sense of rare meaning “difficult.”
They are difficult to separate.
See also in The book of science:
Readings on wikipedia: