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