The Elder Druid was the leading druid of the Juniper Order on Dominaria. Two such druids have been Kolbjorn, his daughter, Kaysa.
Other orders also had Elder Druids, e.g. the nameless Elder Druid of Corondor, and Thelonite Druid.
Mark of the Elder Druid
The Mark of the Elder Druid is a crescent moon mark found on the cheek of a select few individuals on Dominaria. These people have included many of the Carthalion lineage, (though the mark is not strictly hereditary) as well as the druids of the Juniper Order. Only one or two people at a time bear the mark.