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Giant Space Flea from Nowhere specifically requires two parts: It has to be a boss (i.e. the player character(s) fight it), and it has to have no relevance to the plot. Your example seems disqualified on both counts as it is not fought, and evidently these giant beasts are known to exist and are at least "generally present".
It sounds a lot closer to either:
- Always a Bigger Fish - Character A's actions are only loosely relevant; the beast arrived of its own free will, or
- Summon Bigger Fish - Character A, even by accident, compelled the beast to arrive in aid.
The "the main character didn't mean to summon it" part brings to mind a Deus ex Machina if the giant beast helps in any way. If it doesn't, perhaps Big-Lipped Alligator Moment.
Edited by Pumpkin_Cake
Diabolus ex Nihilo is the non-video game-specific supertrope of Giant Space Flea from Nowhere. Unknown Character is for non-villainous examples, I believe?
So, Character A is not a summoner in any way, shape, or fashion. In fact, they have no connection to any of the giant beasts in the game, outside of maybe some incidental dialog in a minor side quest. However, there is a part of the game where they have doubts and their soul gets all muddled, but thanks to the Power of Friendship or whatever, they realize what they have to do and their will becomes ironclad. In doing so, they end up summoning one of the giant beasts, who otherwise was nowhere in sight and has no connection to what's going on, who thanks them for giving them new life thanks to their conviction, and after defeating the enemy in front of them, the giant beast leaves forever.
As the title says, a good guy version of Giant Space Flea from Nowhere. What would the trope be called? Would Giant Flea work?