Problem:
The user would like to have a custom order for a taxonomy dropdown inside a Toolset Form.

Solution:
We can order a taxonomy using a 3rd party plugin without needing any custom coding, term fields or additional work.

This plugin "Category Order and Taxonomy Terms Order" seems to do the work.
https://wordpress.org/plugins/taxonomy-terms-order/

Just install it, then go to your taxonomy link, the "Taxonomy order" link should be below it or at the end of the sublinks. Enter it and reorder your terms. They will, then, be ordered in the form too. Check these screenshots:
- Link for configuration: http://prntscr.com/urah16
- Reorder with drag/drop: http://prntscr.com/urahkw
- Results on the frontend: http://prntscr.com/urair7

Check this reply for a custom solution using views and custom code https://toolset.com/forums/topic/cred-form-taxonomy-order/#post-501029