The focus is very definitely on small animal stuff as that is where most vet nurses end up working - they are very few equine specific vet nurse jobs, yes there are some, but not many.
I am pretty sure there are no equine specific vet nurse degrees, just some which may have a small amount of equine stuff in them.
A vet practice I have done placement at had a nurse with a similar problem - she was working in a mixed practice and prefers the farm animal side of things. But she ended up doing a small animal vet nurse course whilst she was at the practice, so she is now qualified and going to look for courses she can do now to get more qualified in farm stuff.
I have heard on the grapevine that the RVC vet nurse course (we have two, a 4year BSc and a 3yr foundation degree) includes a small amount of optional equine stuff now, but you may want to confirm that with RVC.
Tbh, I think you will struggle to find an equine vet nurse degree, and you may be better finding an equine practice willing to take you on as a student vet nurse and seeing if you can find a course to do whilst working (but I don't know if they even exist, but more likely that a degree).
Bear in mind as well the difficulties with doing a second degree, you won't get much help from student finance. So consider the amount of debt a second degree will get you into compared to a vet nurse salary.