sampo and aha, aaaand another theory i remembered, that has absolutely nothing to do with this drawing, is the one that says aha's form(the human one they took on the astral express) looks like sampo, so thats why sampo doesn't go inside the express, because pom pom would recognize him or something (i don't know if sampo actually entered the express already and that theory is old)
Anyways, IF he didn't enter already, it would make a lot of sense to why in the flashback of pompom saying that aha is the worst trailblazer or something(im going off of memory here bear with me pls) they didn't say the name of them or show the face
I saw this theory on tumblir anyways, if it doesn't make sense blame the poster not me(totally not my fault for explaining everything wrong)
But im still going strong with the puppet/doll sampo theory