According to dictionary.com the word monotypic is an adjective.  When used in biology, it means having only one representative, as a genus with a single species.  Having a single form or member.
Monotypic Cacti
adapted from Tom Glavich
  • Bergerocactus emoryi  (echinocereus emoryi)
  • Blossfeldia liliputana
  • Brasiliopuntia brasiliensis
  • Brachycereus nesioticus
  • Calymmanthium substerile
  • ​Carnegiea gigantea  (Saguaro cactus)
  • ​Cintia knizei
  • Denmoza rhodacantha
  • ​Escontria chiotilla
  • Estopstoopsis dybowskii  (was Austrocephalocereus)
  • ​Geohintonia mexicana
  • Isolatocereus dumortieri
  • Jasminocereus thouarsii
  • ​Leocereus bahiensis
  • Leuchtenbergia principis
  • Mammilloydia candida
  • Mila caespitosa
  • Miqueliopuntia miquelii
  • Obregonia denegrii
  • Ortegocactus macdougallii
  • Rauhocereus riosaniensis
  • Samaipaticereus corroanus
  • Stetsonia coryne
  • Strombocactus disciformis
  • Yavia cryptocarpa
  • Yungasocereus inquisivensis