"What horse did you ride?"
Translation:¿Qué caballo montaste?
The "Rule of Thumb" is you use "Qué" when the next word is a Noun (Qué + sustantivo).
The main exception is when one is choosing between a limited number of alternatives.
i have always been taught that you use "que" for what, and you can use "cual" when in english you can replace the question word "what" with "which". giving a different meaning to the scentence. e.g. "que caballo montaste? 'what horse did you ride?'" (out of the thousands of horses in the world what one did you ride?) and "Cual caballo montaste? 'which horse did you ride?'" (out of the few in the barnyard which horse did you ride specificly)
My objection to this is that 'what horse did you ride' might be colloquially accepted but isn't exactly grammatically correct english, afaik. It should be 'which horse' in both cases - even the what one did you ride in the explanation here sounds bad to my native ears. But you can say 'what type of horse', 'what kind of horse', or something similar for horses in general.
In my notes I changed Preterite to near [or recent] past and Imperfect to [Good] old days just as away to help remember it better.
So most recently is montaste and back when is montabass. Since they used did without a reference to a previous phase of history (another way I think of it) it would imply recent history.
Another way to remember it is that as we get older our memories get less perfect, or imperfect. Sorry you're on your own to remember "Preteritetirietirireert..." ';)