Fugu - pufferfish are very common near Fulung. Iv never seen mass die offs there tho. And Fulung was one of my fav beaches before.
The Japanese are the only ones who regularly eat Fugu. And a few die in Japan every year. Those suckers are deadly poisonous if you eat the wrong parts.
The pig could possibly have come across one and ate it? Same for the rats?
They are not dangerous to swim with though. Unless you are eating one while swimming.
However, TAiwan does have Lion Fish that are dangerous if you touch a spine. Those are uncommon though and not aggressive. Iv swam with them before (carefullY). And there are some stone fish that are also dangerous but again uncommon.
Taiwan also has sea snakes which are rather tame but there is no cure if you are bitten by one (never heard of anyone getting bitten tho).
Shark attacks are rare but have happened. Jellyfish is a big problem at times.