While many of the signs of Qiymah are showing times of increased haram, that does not necessarily mean that in itself a sign is haram.
There can be many reasons, a woman riding a horse would be seen as a sign. What would be the events leading up to an increase of a women riding horses? Some possibilities:
1. Fleeing from war and destruction
2. Obsessive and flagrant display of wealth
3. Lewd behavior resulting in women rushing to men's houses
I do not know if those are the reasons why as qiymah approaches we will see women riding horses but just a thought. My point being I can not find any ayyat or hadith that says a woman riding a horse is haram.
Your opinion is not necessarily wrong. As an opinion we should examine it and see if it is supported by the Qur'an or the Ahadith. But, an opinion by any of us here, my opinion as well as yours is not tafsir and is not to be considered fatwa.
Opinions are good in that they encourage us to think and search. But, we must always be mindfull that some people may think we are making a statement of fiqh which I do not believe either you or myself is trying to do.
Just my opinion Astagfirullah
Opinions are not engraved in stone nor are they unchangeable we each have a right and duty to question each others opinions.