I shall aid you, O Hero from another world!!
This image is unavailable on Donmai. Go to Danbooru or disable safe mode to view (learn more).
Artist's commentary
Isekai Yuusha: A Trustworthy Local Warrior
The native Warriors of this world were scattered in various places by the Ancient Giants, where they dwell hidden... or something.
(https://twitter.com/isiyumi/status/1148206756863942656)
This is pretty much a perfect explanation! Amazing! (Admittedly I sort of drew the whole thing vaguely, so I myself don't know the real answer either.)
(https://twitter.com/isiyumi/status/1148428142169583617)
(said in response to https://twitter.com/nakurubox/status/1148252585075204096)
If I were to further add to this... the idea is that the chariot part (the Chariot Base) is in charge of up to the lower body. So the plan is to swap things out for another mecha in the second half of the series to get a centaur form.
(https://twitter.com/isiyumi/status/1148428876424499201)