Natalis Comes, Mythologiae, 6.23
“Tyndareus allegedly called Helen’s suitors together and had them swear over the testicles of a castrated horse that they would defend Helen….
After they took the oath, they buried the horse on site as Pausanias writes in talking about Laconia. Indeed, it was a common practice of the ancients to take oaths over the testicles of sacrificial animals. This is why when Herakles made a treaty with the sons of Neleus, they swore an oath over the testicles of the sacrificed boar and the dual pledge to provide confirmation of the oath, as Hekataios writes in his Phoroneus.”
Tyndarus dicitur procos Helenae convocasse, qui super equi execti testibus iurarunt se Helenam defensuros … post illud iuramentum Tyndarus equum in eo loco infodit, sicuti scripsit Pausanias in Laconicis. fuit enim antiquorum consuetudo ut super testibus victimarum plerunque iuraretur, cum foedera inter aliquos percuterentur. idcirco ubi Hercules foedus iniit cum liberis Nelei, fide ultro citroque data, sue mactato, super eius testibus et ipse et illi iurarunt atque confirmarunt iuramentum insuper factum, ut scripsit in Phoroneo Hecataeus.