Third time posting this because the cross-platform user-journey for providing feedback is kind of terrible, but I'll save that for a different post.
After learning that the current behaviour of the Managarm mount is intentional, I want to at least suggest an improvement for all cases before I consign it to being unusable.
If you're not familiar with it, the current Managarm mount model looks like this:
For comparison, here's the Fenrir mount (same model, different texture and effects):
At this point I'm willing to be blunt: the Managarm mounts looks terrible. It looks more like the result of a sizing error than a deliberate design. Since first reporting this, I've found multiple instances of players reporting the same dissatisfaction with this mount; several of them on this forum. It's not a good user experience for a player's first impression after earning a new mount to be 'Is this a bug?'.
I don't want to suggest changing the existing mount since that would take away from the people who do like the current size (it does look cute if you're a Lalafell), but for everyone else, I think the right move is to make a separate variant of the Managarm mount that's the same size as the Fenrir mount. That way both user-groups can be happy; win/win.
You could call it the 'Adult Managarm/Elder Managarm', or something along those lines. The distribution method could be similar to - it could be added as a Gold Chocobo Feather reward like the current Managarm, or it could be added as an MGP reward like Fenrir.
I appreciate that this is mostly me shouting into the void at this point, but I was really disappointed by this experience and I'd like to both rectify it and make it so that nobody else has to go through that in the future. I'd like to think someone on the development team might care about this, even if it's a comparatively minor issue.