- Yapi
- Yapping Maw
- Yard
- Yardratian
- Yargonian
- Yarick
- Yarkdao
- Yarmari
- Yarn People
- Yasto Attsmun
- Yatir
- Yautja
- Yayax
- Ychthytonian
- Ydheem
- Yedda
- Yeerk
- Yehat
- Yeki
- Yekokan
- Yekubian
- Yellow Alien (Deep Space Nine)
- Yellow Bulborb
- Yellow Light Aliens
- Yellow Martian (DC Comics)
- Yellow Pikmin
- Yellow Snake
- Yellow Wollywog
- Yerdua Poison-Spitter
- Yeti
- Yeti (FMM-UV 32)
- Yeti (Futurama)
- Yeti (Ribbitking)
- Yeti (Roswell Conspiracies)
- Yevara
- Yevetha
- Yggardis
- Yhemyr Voidshoal
- Yhhanak'ukul'lanjh
- Yhoundeh
- Yidrian
- Yimi
- Yin (Lilo & Stitch)
- Yinchorri
- Yinsect
- Yip Yip
- Yirbek
- Yirmak
- Yivo
- Ylesian White Worm
- Ylymphe
- Yma
- Ymir
- Yobcrab
- Yobshrimp
- Yoda's species
- Yodude
- Yog
- Yog-Sothoth
- Yokian
- Yokoth
- Yolu
- Yondar
- Yootak
- Yor
- Yor (The Lost Regiment)
- Yorg
- Yorgraxxan
- Yorhi
- Yost's species
- Yrashu
- Yridian
- Ysalamir
- Ythe
- Yturi
- Yu'vath
- Yugami
- Yugopotamian
- Yuht
- Yuli
- Yultan
- Yum
- Yumbo
- Yumme
- Yupper
- Yuptar
- Yushyn's species
- Yuuka
- Yuusho
- Yuuzhan Vong
- Yuvernian
- Yuzzem
- Yuzzum
- Yvarema
- Z'nid Bird
- Z'vali
- Z-Ron
- Z-Virus
- Zabrak
- Zadoran
- Zaela Tree
- Zaela tree
- Zaggerak
- Zakdorn
- Zakkeg
- Zakpap
- Zalaaca
- Zalkonian
- Zaltian
- Zamaron
- Zamporiano
- Zan
- Zanan
- Zandir Flower
- Zangan
- Zangan (Aliens vs. Zombies)
- Zangarian
- Zangoid
- Zanibar
- Zanifarian Death Weasel
- Zanon
- Zaoalena
- Zaragoat
- Zaralok
- Zaranite
- Zarathustra Jellyfish
- Zarbi
- Zarbon's race
- Zarglorp
- Zargon Slug
- Zarian
- Zarkhator
- Zaroffian
- Zarolatt
- Zarthan
- Zask
- Zaton
- Zazabi
- Zbornak
- ZeHethbra
- Zeb
- Zebbo
- Zebesian
- Zebesian Dragon
- Zebralope
- Zebranky
- Zedam
- Zeela
- Zeenuian
- Zeevanian Ape
- Zeex
- Zeffliffl
- Zeffo
- Zeioph
- Zeiram
- Zeke Martian
- Zeku
- Zel
- Zelosian
- Zeltron
- Zener
- Zenetan
- Zenn-Lavian
- Zennan
- Zenox
- Zentac
- Zenterran
- Zentraedi
- Zeolosian
- Zeosian
- Zephon's species
- Zerg
- Zerg Drone
- Zerg Hive Cluster
- Zerg Larva
- Zerg Overlord
- Zerg Swarm
- Zergling
- Zeron
- Zeta Reticulan (Starship Invasions)
- Zetan
- Zetarian
- Zetian
- Zetoxian
- Zetton
- Zexx
- Zha'til
- Ziboson
- Zick Slug
- Zig and Zag
- Zigerion
- Zigra
- Zikoro's Species
- Zilkin
- Zillo Beast
- Zin
- Zin Empire
- Zinariya
- Zingkal
- Zinyak
- Zip Zap's species
- Zird
- Zirite
- Zirkonian
- Zirr
- Zirrelian
- Zisian
- Zistle
- Zix, Travoltron, and Tee
- Zizimaak
- Zkrkrxs
- Zllyphan
- Zoa
- Zoa (The Dark Crystal)
- Zoah
- Zoat
- Zodat
- Zoff
- Zoja
- Zolander
- Zolfa-Thuran
- Zolten
- Zoltron
- Zombarian
- Zombie
- Zombie (Space Dandy)
- Zombie Pigman
- Zonama Sekot
- Zone Fighter
- Zongorlu
- Zoni
- Zonian
- Zonn
- Zool
- Zoomer
- Zooplantae
- Zoq-Fot-Pik
- Zor
- Zorellian Jelly Worm
- Zorg Industries
- Zorg Species
- Zorgon
- Zorgon's species
- Zorgon (Zathura)
- Zorgulon
- Zoro
- Zoron
- Zoto
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