Johannesteijsmannia (palm genus) after 19th century Dutch botanist Johannes Teijsmann.

Deuterosmintheurus quinquefasciatus (collembolan)

Gryllotalpa septemdecimchromosomica Ortiz 1958 (mole cricket)

Luopingocoelacanthus eurylacrimalis Wen et al., 2012 (fossil coelacanth)

Pterodactylus crocodiliocephaloides (pterosaur)

Pseudotyrannochthonius octospinosus (pseudoscorpion)

Yunnancoelacanthus acrotuberculatus Wen et al., 2012 (fossil coelacanth)

Griseotyrannus aurantioatrocristatus d'Orbigny and Lafresnaye, 1837 (crowned slaty flycatcher) Longest latinized bird name.

Stigmatodactylus dalagangpalawanicum A.S. Rob, 2016 (orchid)

Weberbauerocereus cephalomacrostibas (Werdermann & Backeberg) Ritter, 1981 (S. American cactus)

Cryptodidymosphaerites princetonensis Currah, Stockey & B. A. LePage 1998 (Eocene fungus)

Prolasioptera aeschynanthusperottetii Mani, 1943 (cecidomyiid fly)

Sirococcus clavigignenti-juglandacearum Nair, Kostichka & Kuntz (butternut canker)

Brachyta interrogationis interrogationis var. nigrohumeralisscutellohumeroconjuncta Plavilstshikov, 1936 (cerambycid)

Anaerobiospirillium succiniciproducens Davis, Cleven, Brown, and Balish, 1976 (bacterium)

Methanothermobacter thermautotrophicus (bacterium)

Micropachycephalosaurus hongtuyanensis Dong, 1978 (pachycephalosaurid dinosaur)

Pseudorhabdosynochus magnisquamodiscum (Aljoshkina 1984) Santos, Buchmann & Gibson 2000 (monogenean flatworm)

Thermodesulfobacterium hydrogeniphilum Jeanthon et al., 2002 (sulfate-reducing bacterium)

Austrocephalocereus dolichospermatichus Buining & Brederoo, 1974 (S. American cactus)

Parapropalaehoplophorus septentrionalis Croft, 2007 (Miocene glyptodont from Chile)

Notiocryptorrhynchus punctatocarinulatus (weevil)

Parastratiosphecomyia stratiosphecomyioides Brunetti, 1923 (stratiomyid fly)

Myxococcus llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogochensis Chambers et al. 2020 (bacterium) Collected in the Welsh parish Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch. Note also the four consecutive 'l's. To save you the counting, there are 63 letters in the epithet. [Genome Bio. and Evo.]

Yamagutiplectognathotrema Paruchin, 1977 (trematode) New name for Plectognathotrema Yamaguti 1971, which is a junior synonym (homonym?). This is the same Satyu Yamaguti who named an even longer genus, below.

Archaeohystrichosphaeridium (fossil dinoflagellate)

Lagenivaginopseudobenedenia Yamaguti, 1966 (Monogenea platyhelminth, a fish parasite) Before this, there was Benedenia Diesing, 1858 (named for Edouard Van Beneden); then Pseudobenedenia Johnston, 1931.

Kimmeridgebrachypteraeschnidium Fleck & Nel, 2003 (fossil dragonfly)
At 31 letters, Kimmeridgebrachypteraeschnidium is, to the best of my knowledge, the longest valid genus names.

Cancelloidokytodermogammarus (Loveninsuskytodermogammarus) loveni Dybowski, 1926 (crustacean)

Crassocornoechinogammarus crassicornis Dybowski, 1927 (amphipod)

Parapallaseakytodermogammarus abyssalis Dybowski, 1927 (amphipod)

Zienkowiczikytodermogammarus zienkowiczi Dybowski, 1927 (amphipod)

Toxophthalmoechinogammarus toxophthalmus Dybowski, 1927 (amphipod)

Siemienkiewicziechinogammarus siemenkiewitschii Dybowski (amphipod)

Rhodophthalmokytodermogammarus cinnamomeus Dybowski, 1927 (amphipod)

Gammaracanthuskytodermogammarus loricatobaicalensis Dybowski, 1927 (amphipod) The above names were proposed by B. Dybowski for amphipods from Lake Baikal; they were invalidated by the ICZN.

Polichinellobizarrocomicburlescomagicaraneus This name is given in Theodore Horace Savory's Naming the Living World, page 83. Alas, he gives no more information about it.

Proterocameroceratidae (Ordovician Endocerida nautiloid)

Stereoplasmoceratidae (Ordovician Orthocerid nautiloid)

Pachycephalosauridae (dinosaur)

Mesembryanthemaceae (ice-plants, now Azioaceae) - Long family names

Saxifraga aizoon var. aizoon subvar. brevifola forma multicaulis subforma surculosa Engler & Irmscher (a variety of the saxifrage, a flowering plant)

Aquilegia flabellata nana pumila alba 'Rama Lama Ding Dong' (a cultivar of dwarf white columbine)

Afipia felis Brenner, Hollis, Moss, English, Hall, Vincent, Radosevic, Birkness, Bibb, Quinn, Swaminathan, Weaver, Reeves, O'Connor, Hayes, Tenover, Steigerwalt, Perkins, Daneshvar, Hill, Washington, Woods, Hunter, Hadfield, Ajello, Kaufmann, Wear and Wenger 1992 (bacterium) The author list may be abbreviated "Brenner et al. 1992" in citations.

Aha ha Menke 1977 (sphecid)

Ia io Thomas 1902 (vespertilionid bat)

Yi qi Xu et al. 2015 (Jurassic theropod) meaning "strange wing"; pronounced "EE CHEE".

Plesiothrips o Girault (thrips)

Adansonia za Baillon 1890 (Madagascan tree)

Automeris io (Fabricius 1775) (io moth)

Dicycla oo (Linnaeus 1758) (moth)

Limnophyes er Saether, 1985 (chironomid) According to Saether, er means "hedgehog."

Stenolicnus ix Wosiack, Montag and Coutinho 2011 (Brazilian catfish) Ix is Mayan for "jaguar", inspired by the fish's coloring.

Io ambondrombeensis (Humbert) B. Nord. 2003 (Asteraceae) but generally considered in genus Senecio.