Dryads are spirits of trees, taking a nymph-like or treefolk form to relate to the humanoids that venture near their forests. They are found on a wide array of different planes. They feature most prominently in Dominaria, Mercadia, Ravnica, and Theros.
Though it may seem unorthodox, dryads have also appeared in Mirrodin, the spirits of the copper trees of the Tangle.
Dryads are established as a type of Nymph, however due to errata problems and their history in Magic, the creature type was preserved, and Therosian dryads are simply of the dual type Dryad Nymph.