So. I won't say specifically about the stock market - but, I remember, once I tried to figure out what would happen to the world if giant monsters from the Pacific Rim or Godzilla appeared. What happens:
1.1. Shipping will be severely affected. Risks will rise sharply.
1.2. Demand for housing, sites for business projects in the depths of continents will grow.
1.3. In general, all activities related to the oceans - deep drilling or fishing - will be under threat.
I think there will be a serious economic crisis, since the entire civilization is now extremely dependent on sea routes of communication, which allow moving large volumes of cargo around the world rather quickly.
Large investments may be made into the defense-industrial complex (defense-industrial complex) if the available weapons are not enough to destroy the new threat.
Potential new refugee crisis - when people from the coasts will be pulled inland. Imagine 120 million Japanese people alone who live in a "monstrous" zone.
So the economy as a whole will be hit hard by the appearance of such monsters (kaijiu). Certain industries (defense industry, construction) may show growth.
Shares of companies whose earnings are expected to decrease due to the appearance of the monster will fall in value. For example, if a monster likes the taste of people who drink Coca-Cola, and it devours a couple of billions of them, then such a massive drop in demand can have a very deplorable effect on the sale of this drink and on the income of the company of the same name. The shares of companies that can make money on weapons to fight monsters, such as aerospace, may go up.