In Gulliver's Travels, how did ordinary animals on the island of Brobdingnag become extremely dangerous to Gulliver?
Because of the enormous size of all living things on the island of Brobdingnag, Gulliver finds himself in mortal danger from animals that humans barely notice. At one point, he is disturbed to find that a common housecat is larger than an ox would be to him, and determines that it will not attack him unless he shows fear. He also finds himself bothered by flies, which taint his food with their spit and toxic feet; the amounts of their poison would be unnoticeable to a relatively-sized person, but to Gulliver, it would be deadly. At another point, he finds himself menaced by wasps:
...after I had lifted up one of my sashes, and sat down at my table to eat a piece of sweet cake for my breakfast, above twenty wasps, allured by the smell, came flying into the room... I dispatched four of them, but the rest got away, and I presently shut my window. These insects were as large as partridges: I took out their stings, found them an inch and a half long, and as sharp as needles.
(Swift, Gulliver's Travels, eNotes eText)
Again, their sting would be an annoyance for a citizen of Brobdingnag, but would stab through Gulliver and cause him great harm. By appealing to his captors, who treat him as a curiosity, and by taking measures to protect himself with makeshift weapons, Gulliver manages to survive and is not killed by, for example, a toad or a dog.