Neptune as a development of the Soviet 3M24 makes an interesting contrast to its Russian counterpart, the Kh-35U, with both aiming for range improvements but doing so in different ways. As the turbojet engine of the original missile was supplied by a Ukrainian manufacturer, the Russians substituted a more efficient, domestically produced turbofan while keeping airframe size largely the same. Due to the greater diameter of the new engine, it is installed in an off-center, semi-submerged configuration as opposed to fully buried as in 3M24 and Neptune.
The latter meanwhile significantly increased length and weight to accommodate more fuel (fair enough, the basic 3M24 is extremely compact). This apparently required the Ukrainians to develop a new booster, which unlike its Russian equivalent is smokeless (or at least reduced smoke). Neptune also has an interesting seeker that appears to use a different principle to the conventional slotted array of the 3M24 (shown on the first photo below, not sure about the Kh-35U). Might be AESA (second photo)?
Where it gets really intriguing is the North Korean KN-19 AShM, which is generally held to be a 3M24 development. If you examine available footage of it more closely, it seems to be less of a direct clone of the Soviet original, the Kh-35U or a "third way" than a Neptune copy though. It retains the buried engine, has the longer airframe and the booster is smokeless! Since Russia seems to be actively supporting the *South* in missile development (KM-SAM/S-350 and more pertinently the Oniks-like supersonic AShM), aid to the North seems unlikely.
Note: I'm not saying the Ukrainians must have necessarily been aiding NK actively, either. Given its proclivities for cyber espionage and similar shady dealings, there are plenty of other ways how it might have obtained the tech. Consider the KN-24 SRBM which is essentially an ATACMS clone - do we seriously expect the US to have lent a hand here? Thought so
EDIT: To illustrate - first pic, very smoky Kh-35U booster exhaust and semi-exposed, short-duct engine; second, KN-19. Compare and contrast with the Nepture photo in Abingdon's post above...
