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- Xanthomyza phrygia — Regent Honeyeater
- Xenoclarias eupogon — Lake Victoria deepwater catfish
- Xenoglaux loweryi — Long-whiskered Owlet
- Xenoophorus captivus — Relict splitfin
- Xenoperdix udzungwensis — Udzungwa Forest-partridge
- Xenophrys brachykolos — Short-legged Horned Toad
- Xenopirostris damii — Van Dam's Vanga
- Xenopoecilus oophorus — Egg-carrying Buntingi
- Xenopoecilus poptae — Popta's Buntingi
- Xenopoecilus sarasinorum
- Xenopus gilli — Cape Clawed Toad, Cape Platanna, Gill's Platanna
- Xenopus longipes — Lake Oku Clawed Frog
- Xenospiza baileyi — Sierra Madre Sparrow
- Xerosecta giustii
- Xipholena atropurpurea — White-winged Cotinga
- Xiphophorus couchianus — Monterrey platyfish
- Xiphophorus gordoni
- Xiphophorus meyeri — Marbled swordtail
- Xylotoles costatus — Pitt Island Longhorn Beetle
- Xyrauchen texanus — Razorback sucker
- Xystichromis nuchisquamulatus
- Xystichromis phytophagus
- Xystichromis sp. nov. 'Kyoga flameback'
Read more about this topic: IUCN Red List Endangered Animal Species