I believe the name dissection resulted in it most likely being a hedgehog. Chestnut-pin a grass hedgehog and keeping in with the seed theme that grass/dark seedot started. Which might further imply chespin is a grass/dark as well.
I've got a quote that embodies you perfectly, but it's seventy-three posts long, has a few massive flowcharts, and lots of Xion-KMA to Me
its Japanese name is "Harimaron". What it is is quite easily identifiable with that alone.
Also, echidnas are "harimogura" (literally "spiky mole"), which sounds pretty similar. So yeah, I'd say it's a chestnut-based echidna, or possibly a porcupine (though if that is the case, it should be mentioned that the naming scheme seems mixed up with Hinoarashi/Cyndaquil, which was named after the Japanese word for porcupine, "yamaarashi", despite looking more like an echidna). On another note, I just realized that this is the first Grass starter we got that isn't a reptile.
EDIT: Noticed other people bringing up hedgehog. For those curious, hedgehogs are "harinezumi" (literally "spiky rat") in Japanese. It could be a hedgehog, but "harimogura" sounding more similar to "Harimaron" made me think that was intentional. BTW... combined with the fox starter, is anyone else getting a Sega vibe from all this?