If you are talking about a full size Type 'C' medium duty short bus you shouldn't have any problems towing a horse trailer.
If you are talking about a small Type 'A' light duty short bus you may run into the problem of running out of GCVW.
As a rule of thumb, since no bus is rated to tow, you will need to use the tow ratings for light duty trucks and vans from the same manufacturer to determine how much GCVWR your bus might have. The tow ratings can be confusing due in large part to differences in engine, transmission, rear gear ratios, and tire sizes.
Be aware that tow ratings are based on best case scenarios. To determine for sure you will need to weigh the empty bus and subtract that weight from the GCVWR to determine your tow rating.
Regardless of the size of the bus you will need to install an electric trailer brake controller. On some of the newer buses with multiplexed electronics and electrical systems it may not be easy to install an electric trailer brake controller. It isn't as simple as it used to be. It isn't just a case of finding the brake light switch and a constant hot lead to power up the controller. If it isn't done correctly you may find the engine won't start, or the transmission won't shift, or the ABS brakes will fail to operate correctly, or any other odd sort of thing will turn on and off every time the brakes are applied.