its not the engine size that matters its the kerb weight.
if you look in your logbook/car manual is will have, a page of weights & measurements you are looking for the cars weight & the towing capacity, then its just a case of knowing the weight of your trialer & horse (including everything you will be wanting to carry). also depends on when you passed your test (pre/post 97)
kerb weight is the cars weight empty (although some makes add a little on in allowance for driver weight), the max towing weight it the max that the car can take but its advisable to not have a trailer weighing more than the tow vehical (car) or in the event of a problem you could get the tail wagging the dog issue.
Yes, but as said above it is the weight that is very important. I used to tow young Stinky and my Bateson Derby with a Nissan Primera which could pull 1300 kilos and was a 2.0 turbo diesel and it never had a problem with going up hill etc. However, as Stinky grew and went up to 570k, I changed to a Vito van which had a tow capacity of 2.1 tonnes and more low down torque which again was a good vehicle for one cob and a small trailer.