Final answer:
Some trolls are creatures who live under bridges, but not all trolls live under bridges. This is an example of an existential fallacy.
Step-by-step explanation:
The statement 'Some trolls are creatures who live under bridges' is true. It is a common concept in folklore that trolls live under bridges and this is represented in stories and fairy tales. However, the statement 'No trolls are creatures who live under bridges' is false. This contradicts the previous statement as it implies that there are no trolls who live under bridges, which is not accurate based on folklore.
This is an example of an existential fallacy. An existential fallacy occurs when someone mistakenly assumes that just because something exists, it must fit specific criteria or characteristics. In this case, the fallacy is assuming that trolls cannot exist if they do not live under bridges. However, that is not necessarily true as there may be other habitats or variations of trolls.