first, be careful with the SPCA/HS in OH specifically. i'm directy involved in the ottawa county / vess arabian horse case in OH and it's a nightmare and the most corrupt HS i have ever in my life dealt with...
that said, it sounds like this horse needs help.
if it all possible as a rescue myself i always encourage people to talk to the owner directly first. ask questions without being accusatory, offer help/assistance, and get as much information as possible. this is not so that you are absolving them of what they did, however it is a good way to get information BEFORE getting any authorities involved and if the owner is open to accepting help, it makes it A LOT easier to help the horse, even if that horse should be euthe'd (which i am not opposed to in severe cases).
that said, once you have info, determine what's next. if the owner won't take the appropriate actions towards the animal, be it vet care, nutritionist, euthanasia, whatever, then present all the information you have to the proper authorities - usually including the state dept of ag and state veterinary office can be helpful.
from there if the owner is not willing to accept help or do the right thing, it's up to the authorities as it is of the utmost importance that the PROPER LEGAL PROCEDURES are followed. if not there is much at risk - from the owner getting the horses back, to crimimal and/or civil charges and worse (and i know as we are dealing with a myriad of complications in the ottawa county ohio case...).
best of luck to you - if i can be of any assistance, please do not hesitate to contact me for anything! and please please keep us posted!