Hiccup could be a bit hesitant. He might not be sure what to do immediately. He would probably look around to see if there are any signs of where the baby came from or if there are any adults nearby. But then his kind heart would probably make him step forward to at least check on the baby's well - being.
He might be shocked at first. After all, finding a baby out of the blue is not an everyday occurrence.
It could be that the baby is from a different species or culture. Hiccup is intrigued and wants to protect it. He may have to learn new things about this baby's origin. Maybe there are some who want to harm the baby, and Hiccup has to fight against them. The story could show Hiccup's growth as a character while dealing with this unexpected discovery of the baby.
Stoick would be quite surprised and a bit worried. As a father, he'd take on the responsibility of caring for his baby son. He might be a bit clumsy at first, not used to dealing with a baby again. The other Vikings would have mixed reactions. Some might find it funny, while others would be more concerned about how to reverse the situation and get their leader back to normal.
Toothless would be extremely confused yet instinctively protective. He would curl around Hiccup to keep him safe from any potential threats. Astrid would be a mix of emotions. She would be worried about Hiccup's well - being and also frustrated that she can't communicate with him in the normal way. The other Vikings would have diverse reactions. The ones who were skeptical of Hiccup's dragon - friendly ways before might see this as a sign that his ideas were wrong all along. However, those who trusted Hiccup would try to support him in this strange new situation. They might start looking for any magical items or wise people in the village who could help reverse the transformation.
Hiccup's friends would be extremely worried. Astrid, being Hiccup's closest friend and love interest, would probably be the first to take action. She would gather the other riders and start planning a rescue mission right away. Snotlout might initially be a bit hesitant, but the thought of losing Hiccup would push him to join in. Fishlegs would use his knowledge of dragons to figure out where Drago might be hiding Hiccup.
Hiccup would probably be very worried. He'd rush to her side immediately. He might start to check her injuries right away and call for help if needed.
Well, he might be initially shocked. He could stand there for a moment, his eyes wide open, trying to process the news. Then, he would probably break into a big smile and hug Astrid tightly, whispering words of joy and excitement.
Their relationship might start with curiosity. Hiccup would be curious about Elsa's ice powers and Elsa about Hiccup's connection to dragons. As they interact more, they start to respect each other's abilities and personalities. This mutual respect turns into friendship, and then gradually into love. The news of the baby would strengthen their bond as they now have a shared future to look forward to. They would support each other more, with Hiccup protecting Elsa during her pregnancy and Elsa using her powers to help Hiccup in his adventures.
Their friends are likely overjoyed at first. Fishlegs might be a bit of a nervous wreck, constantly worrying about Astrid's health and the well - being of the baby. He'd probably be reading every book he can find on dragon - rider pregnancies (if that's a thing in their world). Snotlout would probably make a bunch of inappropriate jokes at first, but deep down he's also excited. Ruffnut and Tuffnut would be their usual chaotic selves, but also show their support in their own unique ways, like bringing strange gifts they think the baby would like.
Astrid might initially be in shock. She could just stand there, frozen, unable to believe that Hiccup is really gone. Her eyes wide open and her body trembling as she tries to process what has just happened.
I think a good way is to think about what kind of personality traits Baby Hiccup would have and build the story around that. For example, if he's particularly brave or mischievous, and how that affects his dragon-training journey.