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In Generation 1 there were only 38 original base cries distributed among 151 Pokemon (160 if you count the crying Missingno). To make all monsters sound unique, developers changed the pitch and length attributes, though some Pokemon still sound the same: Rhydon and Charizard have identical cries, as do Ditto and Poliwag. Zubat and Dragonair share cries with Missingno 069 and 137 respectively.
It seems that specific base cries were created with a particular monster in mind, and then later distributed among other Pokemon according to some common features. Male Nidoran's base cry was later given to Sandshrew and Sandslash for example - all small mammals - and the deleted Pokemon Cactus, who is spiky just like Nidoran Male.
Gyarados' base cry is shared by Ekans line and Onix, all of them are serpentine.
Exeggcute's cry is shared by multi-headed Pokemon such as Dodrio and Dugtrio.
Onix, Farfetch's, Ditto and the Hitmons are the only fully evolved Pokemon that can't learn Hyper Beam in Generation 1.
