Thanks.Treasure 37 is given for completing the penultimate passionate men event. Problem being that if you reach the location of one of them before beating an earlier one, they disappear forever. The most eminently missable of those events is located within the magical horror house and it is the antepenultimate one. If you did get it that one, them the penultimate one is on floor 1 of the dungeon and easy to reach. Treasure 38 is indeed what you thought.
For scene 36, after Plum wakes up (when you reach the last seedbed room) talk to her, then do it again while having all three of the tentacle clothing monsters in your party. You get the monster you asked about by doing this scene.
Luckily I did explore the horror house completely before progressing.
I only somehow missed exactly that junction on F1 leading to the final event with those exploding vending machines.
After your response I searched F1 again and found it.
Also thanks for the info about Plum only waking up after reaching the last seedbed.
I thought I had to progress it somehow like the other events with laying eggs or so.
I feared that progressing passed that door where the spire key is used would lead directly to the final boss.
So I did not go there till I read your reply about the last seedbed.