A very quick use of Google suggests that an Equi trek day trekka weighs about 1100kg. Your handbook will tell you what your car will tow - you can find online versions of Volvo handbooks at https://www.volvocars.com/in/support/manuals/xc60. My XC70 tows 2000kg but it's old. The newer XC60s appear to tow up to 2400kg.
The you'll need to estimate what your horse, tack, water carriers etc weigh and add it all together and add to the weight of the trailer to get the total amount you'll be towing.
As widgeon says you need to find out the towing capacity of your specific model. As you can see from this list Volvo XC60 Towing Capacity & XC 60 Weight Limit Information - All Models it could be as low as 1500kg or as much as 2400kg. If it's the latter you'd probably be fine with an Equi Trek, one horse and all your kit but I don't think you'd manage two unless they're small ponies. If you're struggling for weight you could put all the kit in the car to keep the trailer weight down but that kind of defeats the purpose of having an Equi Trek.
