If you’re referring to the fact that many of them are a bit “pointy,” there are two possible reasons for this.
The first and more scientific reason has to do with Kanto’s terrain and climate. Kanto is a region largely free of natural hazards such as the mountains of Johto, the deserts and volcanoes of Hoenn, the extreme cold of Sinnoh, and the barren wasteland of Unova. (I will stop commenting on Unova when it stops being amusing. Or true.) Instead, Kanto has mild seasons, shorter mountains to the north, lush but not necessarily unnavigable forests, and plenty of fertile land. In short, Kanto’s biomes aren’t exactly extreme, and even though there is some variation (grasslands versus forest versus coast, namely), the transition between them isn’t as abrupt as it is in other regions. Thus, most pokémon adapt to very similar environments and very similar climates, which results in a certain uniformity to the specific adaptations they take on.
Additionally, as for why many of them have similar defense mechanisms (lapras, nidoran, rhyhorn, and so forth are all covered with horns and spikes), it’s essentially because the only real threat most Kantonian pokémon face are other Kantonian pokémon. That abundance of horns, thorns, and other things that discourage physical contact are really the best defense mechanisms against an entire region of pokémon who aim to battle one another, rather than against any specific environmental hazard. This is also why the vast majority of grass-types in the region are also highly poisonous, can exude poison, or are essentially walking balls of things that can choke a kangaskhan.
Alternatively, the second and less scientific explanation is that they’re all descended from Mew, so they’re all basically siblings.
It really depends on whom you ask: the religious sects of Kanto or Kantonian scientists.
The Pokédex According to Bill 