Species Recorded | Taxon | Scientific Name | Common Name | Records | Latest |
| alder fly, lacewing or snake fly | Xanthostigma xanthostigma | a Snake Fly | 4 | 2023 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Xenolytus bitinctus | an insect - hymenopteran | 1 | 2008 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Xestophanes potentillae | a gall wasp | 2 | 2020 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Xiphulcus floricolator | an insect - hymenopteran | 1 | 2002 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Xiphydria | a Xiphydria species (unidentified) | 1 | 2018 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Xiphydria camelus | Alder Wood-wasp | 1 | 1999 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Xorides gravenhorstii | an ichneumon wasp | 1 | 2008 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Xyalaspis petiolata | a small parasitic wasp | 5 | 2003 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Xylocopa violacea | Violet Carpenter Bee | 1 | 2020 |
| beetle | Xantholinus elegans | a rove beetle | 1 | 2019 |
| beetle | Xantholinus linearis | a rove beetle | 19 | 2021 |
| beetle | Xantholinus longiventris | a rove beetle | 42 | 2023 |
| beetle | Xantholinus tricolor | a rove beetle | 1 | 1969 |
| beetle | Xestobium rufovillosum | Death Watch Beetle | 3 | 1993 |
| beetle | Xyleborinus saxesenii | an Ambrosia Beetle | 2 | 2016 |
| beetle | Xyleborus dryographus | an Ambrosia Beetle | 2 | 2019 |
| beetle | Xyleborus monographus | an Ambrosia Beetle | 2 | 2015 |
| beetle | Xylocleptes bispinus | a bark or ambrosia beetle | 5 | 2021 |
| bird | Xema sabini | Sabine's Gull | 35 | 2021 |
| bug | Xanthodelphax straminea | a planthopper | 1 | 2009 |
| bug | Xylocoris (Proxylocoris) galactinus | Hot Bed Bug | 1 | 1900 |
| bug | Xylocoris (Xylocoris) cursitans | a flower bug or bloodsucking bug | 2 | 2002 |
| fish | Xiphias gladius | Swordfish | 4 | 2015 |
| fungus | Xanthoriicola physciae | a fungus | 3 | 2020 |
| fungus | Xerula pudens | a basidiomycete fungus | 9 | 1999 |
| fungus | Xerula radicata | Rooting Shank | 124 | 2022 |
| fungus | Xylaria | a Xylaria species (unidentified) | 4 | 2023 |
| fungus | Xylaria carpophila | an ascomycete fungus | 60 | 2018 |
| fungus | Xylariaceae | a Xylariaceae species (unidentified) | 2 | 2023 |
| fungus | Xylaria hypoxylon | Candle-snuff Fungus | 539 | 2024 |
| fungus | Xylaria longipes | Dead Moll's Fingers | 51 | 2023 |
| fungus | Xylaria polymorpha | Dead Man's Fingers | 135 | 2023 |
| lichen | Xanthoparmelia mougeotii | a lichen | 3 | 2017 |
| lichen | Xanthoria | a Xanthoria lichen (unidentified) | 10 | 2023 |
| lichen | Xanthoria aureola | a lichen | 2 | 2019 |
| lichen | Xanthoria calcicola | a lichen | 12 | 2020 |
| lichen | Xanthoria candelaria s. str. | a lichen | 2 | 1998 |
| lichen | Xanthoria elegans | a lichen | 1 | 1998 |
| lichen | Xanthoria fulva | a lichen | 1 | 2019 |
| lichen | Xanthoria parietina | Common Orange Lichen | 174 | 2023 |
| lichen | Xanthoria polycarpa | a lichen | 20 | 2020 |
| lichen | Xanthoria ucrainica | a lichen | 3 | 2020 |
| mollusc | Xeroplexa intersecta | Wrinkled Snail | 63 | 2014 |
| moth or butterfly - macro moth | Xanthia togata | Pink-barred Sallow | 103 | 2023 |
| moth or butterfly - macro moth | Xanthorhoe | a Xanthorhoe species (unidentified) | 3 | 2023 |
| moth or butterfly - macro moth | Xanthorhoe biriviata | Balsam Carpet | 1 | 2013 |
| moth or butterfly - macro moth | Xanthorhoe designata | Flame Carpet | 591 | 2024 |
| moth or butterfly - macro moth | Xanthorhoe ferrugata | Dark-barred Twin-spot Carpet | 206 | 2023 |
| moth or butterfly - macro moth | Xanthorhoe fluctuata | Garden Carpet | 4825 | 2025 |
| moth or butterfly - macro moth | Xanthorhoe fluctuata fluctuata | a Garden Carpet | 5 | 2007 |
| moth or butterfly - macro moth | Xanthorhoe fluctuata thules | a Garden Carpet (dark f.) | 1 | 2010 |
| moth or butterfly - macro moth | Xanthorhoe montanata | Silver-ground Carpet | 1159 | 2025 |
| moth or butterfly - macro moth | Xanthorhoe montanata montanata | a Silver-ground Carpet | 9 | 2023 |
| moth or butterfly - macro moth | Xanthorhoe quadrifasiata | Large Twin-spot Carpet | 12 | 1995 |
| moth or butterfly - macro moth | Xanthorhoe spadicearia | Red Twin-spot Carpet | 1167 | 2025 |
| moth or butterfly - macro moth | Xanthorhoe spadicearia/ferrugata | a moth | 3 | 2020 |
| moth or butterfly - macro moth | Xestia agathina | Heath Rustic | 7 | 2019 |
| moth or butterfly - macro moth | Xestia baja | Dotted Clay | 16 | 2019 |
| moth or butterfly - macro moth | Xestia castanea | Neglected Rustic | 1 | 2013 |
| moth or butterfly - macro moth | Xestia c-nigrum | Setaceous Hebrew Character | 5861 | 2025 |
| moth or butterfly - macro moth | Xestia ditrapezium | Triple-spotted Clay | 47 | 2020 |
| moth or butterfly - macro moth | Xestia sexstrigata | Six-striped Rustic | 426 | 2024 |
| moth or butterfly - macro moth | Xestia stigmatica | Square-spotted Clay | 3 | 2006 |
| moth or butterfly - macro moth | Xestia triangulum | Double Square-spot | 1309 | 2025 |
| moth or butterfly - macro moth | Xestia xanthographa | Square-spot Rustic | 4559 | 2024 |
| moth or butterfly - macro moth | Xylena exsoleta | Sword-grass | 3 | 1981 |
| moth or butterfly - macro moth | Xylena vetusta | Red Sword-grass | 27 | 2020 |
| moth or butterfly - macro moth | Xylocampa areola | Early Grey | 1992 | 2024 |
| plant | Xanthium spinosum | Spiny Cocklebur | 4 | 2009 |
| plant | Xanthium strumarium | Rough Cocklebur | 6 | 2000 |
| plant | Xanthocyparis nootkatensis | Nootka Cypress | 3 | 2000 |
| spider | Xysticus | a Xysticus species (unidentified) | 11 | 2023 |
| spider | Xysticus acerbus | a crab spider | 2 | 2019 |
| spider | Xysticus cristatus | a crab spider | 89 | 2022 |
| spider | Xysticus erraticus | a crab spider | 7 | 2000 |
| spider | Xysticus kochi | a crab spider | 9 | 1999 |
| spider | Xysticus lanio | a crab spider | 2 | 2020 |
| spider | Xysticus ulmi | a crab spider | 1 | 1994 |
| springtail | Xenylla | a Xenylla species (unidentified) | 1 | 2016 |
| springtail | Xenylla grisea | a springtail (Collembola) | 1 | 1925 |
| true fly - | Xanthogramma | a Xanthogramma species (unidentified) | 2 | 2016 |
| true fly - | Xylophagus ater | a fly | 1 | 1998 |
| true fly - hoverfly | Xanthandrus comtus | a hoverfly | 5 | 2014 |
| true fly - hoverfly | Xanthogramma citrofasciatum | a hoverfly | 22 | 2023 |
| true fly - hoverfly | Xanthogramma pedissequum (sensu lato) | a hoverfly | 138 | 2025 |
| true fly - hoverfly | Xanthogramma pedissequum (sensu stricto) | a hoverfly | 1 | 2024 |
| true fly - hoverfly | Xylota segnis | a hoverfly | 145 | 2024 |
| true fly - hoverfly | Xylota sylvarum | Golden-tailed Hoverfly | 41 | 2023 |
| true fly - hoverfly | Xylota tarda | a hoverfly | 2 | 2008 |
| true fly - hoverfly | Xylota xanthocnema | a hoverfly | 3 | 2022 |
| true fly - picture-winged, conopid or stilt-legged fly | Xyphosia miliaria | a gall fly | 9 | 2016 |
| true fly - robber, empid or dolichopid fly | Xanthochlorus | a Xanthochlorus species (unidentified) | 2 | 2006 |
| true fly - robber, empid or dolichopid fly | Xanthochlorus galbanus | a dolichopodid fly | 2 | 2020 |
| true fly - robber, empid or dolichopid fly | Xanthochlorus luridus | a dolichopodid fly | 4 | 2006 |
| true fly - robber, empid or dolichopid fly | Xanthochlorus ornatus | a dolichopodid fly | 1 | 2000 |
| true fly - robber, empid or dolichopid fly | Xanthochlorus tenellus | a dolichopodid fly | 1 | 1995 |