(*disclaimer- I'm from the US and when I see "dissertation," I assume you are talking about a project at the PhD level.)
Do you have to collect your own data as a program requirement? Hmmm...if you are as limited in your data collection options as you describe above, I think your best bet might be to recruit owners of laminitic horses to keep health logs over a specified period of time, tracking overall health, hoof health, foot care practices, exercise, and diet. Then maybe you'd be able to do a correlational study of how care and diet relate to hoof health? It's hard if you can't really propose an actual treatment/intervention and then divide horses into treatment and control groups, so you'd have to be pretty creative going in about what hypotheses you have regarding how care & diet affect health.
Without knowing the full requirements of your program, it's hard to know how large a study is the norm, but if it's too small a sample, you run into problems with being able to detect any statistically significant effects even if an intervention or treatment is actually working.
I often tell graduate students- as I was told myself- that unless you have a really large research grant, your best course of action is to look for an existing dataset that allows you the opportunity to pose your own research question without having to collect your own data. I know it seems a lot sexier to collect your own data, but remember- a good dissertation is a finished dissertation. Trying to collect data on your own, without major financial backing, sets you up to have lower quality data than someone who has a great dataset and can find an interesting question to ask of the data. Does your advisor/major professor have data on this topic?
Also- the above is predicated on the assumption that you have other opportunities to participate in a project that gives you experience with original data collection. Depending on what you want to do in the future, you will probably need to be sure you can point to other projects where you've done this, if it's not going to be part of your dissertation.
Don't know if this helps, but good luck with your project.