Oshawott is kinda random, since Unova is supposed to be so far away. And Rowlet is just.... eeehh.
1) japan has otters and owls
2) Samurott is samurai based and was always meant to be of Japanese decent.
Sugimori:“Then we do another division of roles for the evolved forms. This time around — because the Unova region has a racially diverse human population — we decided to make the starter Pokémon’s evolutions symbolize the cultures of Japan, the West, and China. Snivy’s final evolution, Serperior, was designed with the concept of a European knight in mind, kind of like Lady Oscar from The Rose of Versailles. Tepig’s final evolution, Emboar, stands up on two legs and becomes humanoid, so we imagined him as Zhang Fei from the Chinese Romance of the Three Kingdoms. After we decided on the Fire and Grass types, we still had to decide on Oshawott’s evolutions — so we made them into Japanese Samurai. When we came up with the idea that Dewott can remove the shell from his stomach and use it as a weapon, we felt we’d succeeded in making him a Samurai.”
3) Decidueye is the practically the only ninja among the grass type starters, given that their sticking to the normal 3 elements and one the covert jobs of a ninja was to stealthily eliminate samurai, often at the request of other samurai, its pretty obvious why it was chosen as the only other ninja among the starters is a water type, while the only other starter that was samurai-like was a grass type which doesn't fit as well with the dynamic.
4) Typhlosion is most likely the Yokai of the group, namely a Mujina
you can even see the resemblance
might also have a bit of monk mixed in and yes feniken might have fit better thematically as yokai, but they also seem to have been avoiding 1st gen, 4th & 8th gen starters while keeping a gap between generations most likely to have the starters cater to different age groups that where not already attended by home/sw/sh/bd/sp while still remaining in theme(gen 2, gen 5, gen 7) gf also seems to try and keep all non-mythical pokemon around in each generation so theres a good chance that the 3ed and 6th gen starters will be available between BD/SP/legends
5) each one represents part of the land; Samurott represents rivers, Decidueye represents the forest & Typhlosion represents the volcanos