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- UakariGenus of New World monkeys
- UintatheriumExtinct genus of mammals
- Ultrasound avoidanceAnimal reflex to escape or avoid echolocating predators
- Umbrella octopusFamily of molluscs
- Uncinaria stenocephalaSpecies of roundworm
- UnenlagiaUnenlagia (meaning "half-bird" in Latinized Mapudungun) is a genus of dromaeosaurid therop
- UnenlagiidaeUnenlagiidae is a proposed family of eumaniraptoran paravians that includes the subfamilie
- UnenlagiinaeExtinct subfamily of dinosaurs
- UnguinychusUnguinychus (meaning "claw claw") is an extinct genus of drepanosaurid reptiles from the L
- UngulateGroup of mammals that walk on the tips of their toes or hooves
- Unicolored blackbirdThe unicolored blackbird (Agelasticus cyanopus) is a species of bird in the family Icterid
- Uniform swiftletThe uniform swiftlet (Aerodramus vanikorensis), also known as the Vanikoro swiftlet or low
- UnionidaOrder of bivalves
- United States Army Pigeon ServiceThe United States Army Pigeon Service (a.k.a. Signal Pigeon Corps) was a unit of the Unite
- Universal Declaration on Animal WelfareProposed inter-governmental agreement on animal rights
- UnkenreflexUnkenreflex – interchangeably referred to as unken reflex (Unke is the German word for fir
- UnktaheelaUnktaheela is an extinct genus of polycotylid plesiosaur from the Late Cretaceous Sharon S
- Upemba lechweSubspecies of mammal
- Upemba National ParkUpemba National Park (French: Parc national d'Upemba) is a large national park in Haut-Lom
- Urechis unicinctusSpecies of annelid worm
- UrmetazoanHypothetical last common ancestor of all animals
- UrocyonGenus of carnivores
- UromastyxGenus of lizards
- UromyiasUromyias is a genus of small Andean, tyrant flycatchers known as tit-tyrants. They were fo
- UromysUromys is a genus of rodents found in Melanesia and Australia. They are known as the giant
- UropygiOrder of arachnids known as whip scorpions
- Ursid hybridBear hybrids
- UrsidaTaxonomic group of carnivorans within Arctoidea
- UrsinaeSubfamily of bears
- Ursine tree-kangarooThe ursine tree-kangaroo (Dendrolagus ursinus) is a long-tailed, furry, bear-like mammal f
- Ursus (mammal)Genus of bears
- UruguaysuchidaeUruguaysuchidae is a family of notosuchian crocodyliforms that lived in South America and
- USFC PhalaropeUSFC Phalarope was an American fish culture and fisheries science research vessel that ope
- Ussuri dholeThe Ussuri dhole (Cuon alpinus alpinus), also known as the Eastern Asiatic dhole and the C
- Utah prairie dogThe Utah prairie dog (Cynomys parvidens) is the smallest species of prairie dog endemic to
- UtaherpetonUtaherpeton is an extinct genus of lepospondyl amphibian from the Carboniferous of Utah. I
- UtahraptorGenus of dinosaur from the Early Cretaceous
- UtaniaUtania is a genus of flowering plants belonging to the gentian family (Gentianaceae), the