Snowball is only dangerous to Napoleon--and perhaps to the humans of the area that surrounds Animal Farm. Snowball is Napoleon's main rival for leadership of the farm. He is more intelligent than Napoleon, and his ideas are progressive and based upon the betterment of the farm and its animals. Snowball also proves to be a brave fighter and tactician, and he leads the animals to victory in the Battle of the Cowshed. This makes him a threat to Napoleon, who fears that Snowball may eventually become the dominant leader.
I do not think that anyone on Animal Farm other than Napoleon would think Snowball is dangerous. But, to Napoleon, Snowball is dangerous because he is a rival for power.
Snowball is the only one who seems to have the skills that are needed to compete with Napoleon to rule the farm. He is good at thinking up new ways to improve the economy of the farm, for example. He is also a skilled military leader as we see at the Battle of the Cowshed.
Because of these things, he is admired just as much as Napoleon is. This is what makes him dangerous.
I find that linking the Animal Farm characters to their Russian Revolution counterparts helps me to think through why they do what they do.
Snowball represents Leon Trotsky. He was a celebrated military mind, and he instituted many committees. This is a dangerous place when the committees don't necessarily achieve anything.
Snowball followed in these footsteps. He tried to institute these committees: re-education, planning and development, and production. What has made that dangerous for Snowball is that he spent all this time thinking and for what? Working with incompetent animals? This is dangerous for him, but it is also dangerous to another authority figure because it shows that he can think and plan.
Snowball would have been prooved dangerous to none other than Napoleon. Napoleon finds difficult to compete the ideas of Snowball. It threatens him that he may be the only obstacle in his way towards leadership. He finds Snowball coming out with the ideas of the windmill and posses all the leadership qualities. The above stated factors make Snowball dangerous for Napoleon