Could be a Glyptodon, we have yet to see pokemon that truly look like pleistocene and Miocene animal such as Smilodon, Ground Sloth, Elasmontherium, Indrotherium etc
O wait, we have Mamoswine (Mammoth), Slaking (Ground Sloth or Gigantopithecus), maybe saber tooth cat (Raikou)
I think it is because of that , that is why fennekin exist and they want a non-legendary Fire/Psychic type to be readily available and do not have to resort to darmanitian Zen Mode or Victini.
I dun think that they will use Dark type of Chespin, because when you look at all Grass starter that have dual type -
Venusaur - Poison, Torterra - Ground, They have fewer weakness than before.