The socialization period for puppies ends between 12 and 16 weeks. Your puppy should have as many good experiences as possible before he's 16 weeks. Socialization has no end, however, so continue to expose your adolescent dog to new situations.
Today Talos had the good fortune of meeting a horse. A good friend of mine has a calm, good natured, senior horse (and a puppy) and invited us over for a visit with Spunky. Talos often runs a little short on courage, so I was curious.
To keep him safe, I kept Talos on leash, and tried to keep him in front of the horse. I was ready with delicious treats and a calm voice, in case Talos was spooked. As you can see from the video, the horse is calm, and Talos is awfully curious. I was happy to see curiosity, not fear, from Talos. He was appropriately cautious around the horse, but not overly.
We'll chalk this up to one more good experience for Talos. Every good experience builds on the last and the more novel things Talos sees, the fewer things he'll be bothered by later in life, even if he's never experienced it before.
If you want to get your dog out more, make sure you have (1) fantastic food treats (chicken, steak, salmon) and (2) lots of patience. If Talos had been afraid, I would have walked him away from the horse, settled him down with a few treats and a calm voice, then rewarded him for just looking at the horse. And if he was simply too afraid, we would have left the situation completely. Keep an eye on your dog, watch him for signs of stress, and proceed when he tells you he's comfortable.