Prior

1. Advance; previous; coming before.
2. Former, previous.

The etymological antonym is ulterior (from Latin; compare primate/ultimate for “first/last”). This is now no longer used, however, and there is no corresponding antonym. Typically either subsequent or posterior is used, but these form different pairs – precedent/subsequent and anterior/posterior – and are more formal than prior. When an opposing pair is needed, these can be used, or other pairs such as former/latter or previous/next.

anterior

former

posterior