If you enter a bear's den, especially if it has cubs, the bear will likely attack you.

If you enter the territory of a swan, especially during nesting season, the swan might attack you.

If a foreign object enters some animal's body, the immune system may attack that object.[0] Allergy might be related to the immune system misidentifying allergens.

Squirrels can be surprisingly territorial.

Ants have wars. [1]

This is not surprising, since the consequences of territory being compromised can be severe. For instance, in this case [2], the territory was compromised through deception, like pretending to be one of them, and it led to the severe weakening or death of the whole colony through the mass devouring of their offspring.

[0]: https://en.wikipedia.org/wiki/Foreign_body_reaction

[1]: https://en.wikipedia.org/wiki/War_in_ants

[2]: https://www.reddit.com/r/Damnthatsinteresting/comments/123ke...