Actually, autochthonous is not a rare word, but I used to see it infrequently enough that I had to keep looking it up.I'm surprised it was in the Peacock list.
autochthon
n.,
pl. 3thons or 3tho[nes# 730! ncz#8 5Gr autochthon, sprung from the land itself < autos, self + chthbn, earth: see HOMAGE6
1 any of the earliest known inhabitants of a place; aborigine
2 any indigenous animal or plant
au[toch4tho[nous 730! n!s8 or au[toch4tho[nal
adj.
au[toch4tho[nous[ly
adv.