Yes you should have your horse vaccinated against tetanus. Tetanus is caused by an organisim, Clostridium tetani, that lives in the soil, this organisim can enter the horse through a tiny puncture wound that could go un-noticed and once the disease has taken hold it usually proves fatal.
The other common equine disease to vaccinate against is equine influenza.
If you intend to compete with your horse you'll find that you will usually require proof of flu vaccination. Equine governing bodies take this precaution as a way of minimising the risk of infectious diseases at events where horses come into contact with one another.
If your horse is going to mix with a number of horses or ponies from different places it's a good idea to have him vaccinated against equine flu.
If you board at a livery yard you might be asked to vaccinate against equine flu as a precaution against infectious disease. Even if you are not asked to do so it is probably a good idea as the introduction of a new horse could expose your horse to the virus.
However if your horse or pony is never going to come into contact with new horses then it's risk of exposure to equine flu is going to be very low and you probably don't need the flu vac.
