If such a creature exists, it would be a pirhana...
It is true that a troll under a bridge plays an important part in one particular saga, but as far as I know, it's only in that one saga it lives under a bridge.
A "troll" is a creature from Norwegian folklore, that was malicious, stupid and that existed in nature. There were several forms of trolls, the "classical trolls" lived inside the mountains, while other forms, like "nøkken" and "draugen", lived in rivers or ponds in the forests, or ate sea. It was the 19th century drawer Theodor Kittelsen who established the shape of the "classical troll" in his illustrations of the folklore sagas, collected by Asbjørnsen and Moe (the Norwegian equivalents of the brothers Grimm).
If you google for "troll kittelsen" you will find some of Kittelsen's classical illustrations. My objections to his illustrations are that the trolls are generally shown too small (they are literally the sizes of mountains) and that his trolls are "forest trolls" as found in the south-east parts of Norway.
There is a saga about a bunch of trolls in the nortern parts of Norway, that are "documented" in the landscape. One day, the troll "Vågakallen" felt a bit, well, horny. He knew there was bunch of maidens nearby, so quite naturally he went off to make there aquaintance. The maidens weren't awfully interested in any close encounter with this raving madman, so they ran off towards the south. The whole thing was about to take on an ugly twist, when the troll king "Suliskongen" got aware of the situation. "Vågakallen" was just about to fire an arrow after the fairest maid, "Lekamøya", but "Suliskongen" managed to throw his hat to intercept the arrow. The arrow went straight through the hat but missed the maiden. Just as this happened, the sun rose above the horizon, and trolls don't stand to sunlight. The whole bunch petrified.
"Vågakallen" became one peak of the Lofoten mountains west of Bodø. "Suliskongen" petrified some 100 km due east of Bodø, and is now a peak near the Swedish border, near the village Sulitjelma (the short form of the name is Sulis). The bunch of the maids are known as the "Seven Sisters" peaks near Sandnessjøen, while the hat with the hole through it became "Torghatten" island near Brønnøysund. The fair maid "Lekamøya" made it some 100 km south of Brønnøysund, at the border between provinces Nord-Trøndelag and Nordland, before she petrified. All the town and village names should be easy to find on a decent map of Norway. They are all there to see, if you travel by the coastliner.
so the common denominator of trolls, is that they are stupid, malicious, and don't survive getting exposed to scrutiny in bright sunlight.
Rune