The transition for kobolds seems to have been between 2E and 3.0:
In 2E, they have "scaly hides" "smell of damp dogs" "two small horns" "rat-like tail" and a language that sounds like "small dogs yapping": no reference EDIT:to eggs, or being reptilian. Strong elements of dog and rat, but with little horns and hide that could be read either as mammalian or reptilian. The picture in the MM shows pinkish mammalian skin and whiskers (though see Voadam's post immediately above!).
In 3E, they are explicitly "reptilian" with "scaly skin" and oviparous ("egg") and they speak Draconic "with a voice that sounds like that of a yapping dog". The picture shows green lizard-like skin.
EDIT: as pointed out below, eggs predate this. I am wrong.
In 2E, they have "scaly hides" "smell of damp dogs" "two small horns" "rat-like tail" and a language that sounds like "small dogs yapping": no reference EDIT:
In 3E, they are explicitly "reptilian" with "scaly skin" and oviparous ("egg") and they speak Draconic "with a voice that sounds like that of a yapping dog". The picture shows green lizard-like skin.
EDIT: as pointed out below, eggs predate this. I am wrong.
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