Species Recorded | Taxon | Scientific Name | Common Name | Records | Latest |
| alder fly, lacewing or snake fly | Xanthostigma xanthostigma | a Snake Fly | 3 | 2013 |
| 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 | 1 | 2016 |
| 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 (Xylocopa) violacea | Violet Carpenter Bee | 1 | 2020 |
| beetle | Xantholinus (Purrolinus) tricolor | a rove beetle | 1 | 1969 |
| beetle | Xantholinus (Xantholinus) linearis | a rove beetle | 8 | 2016 |
| beetle | Xantholinus (Xantholinus) longiventris | a rove beetle | 21 | 2018 |
| beetle | Xestobium rufovillosum | Death Watch Beetle | 3 | 1993 |
| beetle | Xyleborinus saxesenii | an Ambrosia Beetle | 2 | 2016 |
| beetle | Xyleborus dryographus | an Ambrosia Beetle | 1 | 2018 |
| beetle | Xyleborus monographus | an Ambrosia Beetle | 2 | 2015 |
| beetle | Xylocleptes bispinus | a bark or ambrosia beetle | 4 | 2003 |
| bird | Xema sabini | Sabine's Gull | 6 | 2012 |
| 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 | 3 | 1984 |
| fungus | Xanthoriicola physciae | a fungus | 2 | 2008 |
| fungus | Xerocomellus engelii | a fungus | 4 | 2014 |
| fungus | Xerula pudens | a basidiomycete fungus | 9 | 1999 |
| fungus | Xerula radicata | a fungus | 17 | 2011 |
| fungus | Xerula radicata | Rooting Shank | 103 | 2014 |
| fungus | Xylaria carpophila | an ascomycete fungus | 60 | 2018 |
| fungus | Xylaria hypoxylon | Candle-snuff Fungus | 344 | 2021 |
| fungus | Xylaria longipes | Dead Moll's Fingers | 45 | 2016 |
| fungus | Xylaria polymorpha | Dead Man's Fingers | 126 | 2018 |
| lichen | Xanthoparmelia mougeotii | a lichen | 3 | 2017 |
| lichen | Xanthoria | a Xanthoria lichen (unidentified) | 5 | 2018 |
| lichen | Xanthoria aureola | a lichen | 1 | 2014 |
| lichen | Xanthoria calcicola | a lichen | 11 | 2019 |
| lichen | Xanthoria elegans | a lichen | 1 | 1998 |
| lichen | Xanthoria fulva | a lichen | 1 | 2019 |
| lichen | Xanthoria parietina | Common Orange Lichen | 156 | 2019 |
| lichen | Xanthoria ucrainica | a lichen | 2 | 2010 |
| moth or butterfly - macro moth | Xanthia togata | Pink-barred Sallow | 89 | 2021 |
| moth or butterfly - macro moth | Xanthorhoe | a Xanthorhoe species (unidentified) | 1 | 2018 |
| moth or butterfly - macro moth | Xanthorhoe biriviata | Balsam Carpet | 1 | 2013 |
| moth or butterfly - macro moth | Xanthorhoe designata | Flame Carpet | 511 | 2021 |
| moth or butterfly - macro moth | Xanthorhoe ferrugata | Dark-barred Twin-spot Carpet | 202 | 2019 |
| moth or butterfly - macro moth | Xanthorhoe fluctuata | Garden Carpet | 3877 | 2021 |
| moth or butterfly - macro moth | Xanthorhoe fluctuata subsp. fluctuata | a Garden Carpet | 5 | 2007 |
| moth or butterfly - macro moth | Xanthorhoe fluctuata subsp. thules | a Garden Carpet (dark f.) | 1 | 2010 |
| moth or butterfly - macro moth | Xanthorhoe montanata | Silver-ground Carpet | 1039 | 2021 |
| moth or butterfly - macro moth | Xanthorhoe montanata subsp. montanata | a Silver-ground Carpet | 5 | 2014 |
| moth or butterfly - macro moth | Xanthorhoe quadrifasiata | Large Twin-spot Carpet | 12 | 1995 |
| moth or butterfly - macro moth | Xanthorhoe spadicearia | Red Twin-spot Carpet | 927 | 2021 |
| moth or butterfly - macro moth | Xanthorhoe spadicearia/ferrugata | a moth | 2 | 2016 |
| moth or butterfly - macro moth | Xestia agathina | Heath Rustic | 6 | 2009 |
| moth or butterfly - macro moth | Xestia baja | Dotted Clay | 15 | 2014 |
| moth or butterfly - macro moth | Xestia castanea | Neglected Rustic | 1 | 2013 |
| moth or butterfly - macro moth | Xestia c-nigrum | Setaceous Hebrew Character | 4537 | 2021 |
| moth or butterfly - macro moth | Xestia ditrapezium | Triple-spotted Clay | 44 | 2017 |
| moth or butterfly - macro moth | Xestia sexstrigata | Six-striped Rustic | 322 | 2020 |
| moth or butterfly - macro moth | Xestia stigmatica | Square-spotted Clay | 3 | 2006 |
| moth or butterfly - macro moth | Xestia triangulum | Double Square-spot | 1118 | 2021 |
| moth or butterfly - macro moth | Xestia xanthographa | Square-spot Rustic | 3472 | 2021 |
| 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 | 1534 | 2021 |
| plant | Xanthium spinosum | Spiny Cocklebur | 4 | 2009 |
| plant | Xanthium strumarium | Rough Cocklebur | 4 | 2000 |
| plant | Xanthocyparis nootkatensis | Nootka Cypress | 3 | 2000 |
| spider | Xysticus | a Xysticus species (unidentified) | 4 | 2016 |
| spider | Xysticus acerbus | a crab spider | 1 | 2019 |
| spider | Xysticus cristatus | a crab spider | 86 | 2018 |
| spider | Xysticus erraticus | a crab spider | 7 | 2000 |
| spider | Xysticus kochi | a crab spider | 9 | 1999 |
| spider | Xysticus lanio | a crab spider | 2 | 2012 |
| spider | Xysticus ulmi | a crab spider | 1 | 1994 |
| springtail | Xenylla | a Xenylla species (unidentified) | 1 | 2016 |
| 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 | 2019 |
| true fly - hoverfly | Xanthogramma pedissequum | a hoverfly | 110 | 2019 |
| true fly - hoverfly | Xylota segnis | a hoverfly | 115 | 2020 |
| true fly - hoverfly | Xylota sylvarum | Golden-tailed Hoverfly | 35 | 2020 |
| true fly - hoverfly | Xylota tarda | a hoverfly | 2 | 2008 |
| true fly - hoverfly | Xylota xanthocnema | a hoverfly | 2 | 2004 |
| 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 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 |