Presumably you are using a stables of some kind who are providing a commercial service. That being the case they should be able to provide you with suitable risk assessments. If they can't then I would seriously suggest you don't use them. I'm sure that they should be registered with one asociation or another if they are selling pony trekking to the public. Look at their paperwork to see if it has a licence number or affiliation on it. If they are a pukka organisation they will have this.