I do want to put this disclaimer out there, I've not used any of these companies nor am I being paid by any of them. I just thought it would be something interesting to post about since summer is coming up. Maybe you haven't figured out what you want to do in your time off. Maybe you're due for a vacation. Here are some options that you can look into.
If you want to focus more on the physical benefits of working with horses, Argentina has a good project in place for you. They focus on movement, posture, and muscle control of the rider, to aid physical recovery. Another plus about this one is that you don't have to commit to a full month.
Maybe your Spanish isn't so good. Then Aldeas de Paz in Venezuela might be more your speed. They also work with horses, this time with disadvantaged youth, and they allow volunteers with limited Spanish. You can also go through Frontier to set this placement up.
Working with horses might not be all that you want to do. In Ecuador, you work in an orphanage in the morning and then assist with "Horse Guided Empowerment" lessons in the afternoon. The cool thing about this one is that the money you pay ($500 per month) to volunteer there, actually finances clients who can't pay.
Speaking of disabled, Cape Town offers equine therapy to physically disabled children. This program have been in place since 1983 so it is well established. If you're looking for something to do for the majority of the summer, this might be it. The time span can be anywhere from 8-26 weeks.
I know not everyone is interested in working with horses. The Institute for Cultural Ecology has an animal assisted therapy placement for you lovers of other animals. Some of the animal shelters that you can intern at have visiting pet programs, where they take the shelter animals to nursing homes and health care facilities. I'm not sure which locations offer this but the company has placements in Fiji, New Zealand, France, Thailand and Napal to name a few locations.
And of course, for some of you the US is "abroad". Here's a list of Animal Assisted Therapy volunteer, internship, and educational opportunities available in the United States.
***Update: I just got an e-mail back from CISabroad. They advertise that they can create internships to fit you so I had asked about animal assisted therapy and this was their response:
Hi Sarah. It's very likely, but we can't know for sure until we know your credentials and can begin the search process in our placement networks! We customize each internship based on your background, relevant skills and what's available in our host locations. That may be a new placement type for us, but nothing is impossible! In the case we can't find that specific type of placement, you don't lose anything. It might be easier to correspond via email/phone, so please visit the following link to start the process:http://www.cisabroad.com/contact-advisor
They too have placements all over the world, so check them out if you don't see what you want already listed.