Greek Grammar

Ho delphin dolphin. tou delphinos. Ho Phorkyn Phorcys. tou Phorkynos. Those ending in -in and -yn form the genitive by adding -os. Ho leōn lion. tou leontos.
Ho Theōn Theon. tou Theōnos. Ho Sarpēdōn Sarpedon. tou Sarpēdonos. Ho Poseidōn Poseidon. tou Poseidōnos. Ho Xenophōn Xenophon. tou Xenophōntos. Ho koitōn bedroom. tou koitōnos. Ho agōn contest. tou agōnos. Ho cheimōn winter/storm. tou cheimōnos.
For those ending in -ōn that are barytone: those that are paronyms derived names decline through -ōnos, such as Kikerōn Cicero, Kikerōnos. Those that are verbal or common in gender decline through -onos, such as Philēmōn, Philēmonos; beltiōn better, beltionos; sōphrōn prudent, sōphronos. Those that have feminine forms in -aina decline through -nt-, such as therapōn attendant, therapontos, because it is therapaina, except for Lakōn Laconian, Lakōnos, for which there is however Lakaina.
Those ending in -ōn that are oxytone and perispōmena circumflex-accented decline through -ōntos, such as Xenophōn, Xenophōntos, except for Poseidōn, Poseidōnos and taōn peacock, taōnos. Those that are not perispōmena and end in -dōn decline through -onos, such as Sarpēdōn, Sarpēdonos, except for Sidōn, Sidōnos.