Linked from
The 8 pages that link to Antiquark, each with the reason it gives.
Quark modelBroader topic: Antiquarks pair with quarks to form mesons in the model.
MesonBroader topic: An antiquark pairs with a quark in the conventional meson structure.
AntiprotonBroader topic: Two anti-up quarks and one anti-down quark make an antiproton.
AntineutronBroader topic: Three antiquarks make up an antineutron.