Good day, dear friends.
At the moment I'm at a dead end, working out ideas for my fan fiction (which may never see the light of FimFiction), and I want to ask for help.
The crux of the problem: One of the main characters is one of many slave dragons and belongs to a family of unicorn druids (yes, I plan to add spirits, but in my own way). The background of his getting to this particular family has not yet been chosen, but I prefer the path of the gladiator, because there are so many more options for plot development. The dead end was the question: "How did dragons become slaves to ponies?"
Based on pure logic, dragons are extremely dangerous to any other species, even if it has magic. And then this kind of alpha predators suddenly becomes under the hoof of a not very strong race of unicorns. By the way, a small question – How do you feel about the species difference in the dragon race? (drakes, wyverns, etc.)
There are two possible answers to the dead-end question so far:
1) In some war, ponies found a way to deprive dragons of one of their main abilities - to breathe fire, after which victory over them and subsequent enslavement become not such a big problem.
2) Dragons have existed for a long time as disparate tribes, but a crisis occurred that forced them to choose a single leader – the Dragon Lord, and accordingly, appoint a test for anyone who wants to take this place. One of these dragons, knowing his abilities, decided to cheat and made a deal with unicorns, according to which, after the end of the tests, all the opponents of the new ruler must either die or go into slavery. (Who played in "Lord of the rings: Shadow of Mordor", remember the character named Ratbag)
These are the ideas I have, I would like to hear your opinion and possible suggestions.
Thank you all very much in advance.

Just some random early morning thoughts:

- I like the second option you have proposed.

- Could be there was food scarcity at some point in the past for dragons. The unicorns, knowing the strength of dragons, yet needing that same strength for manual labour, took the dragons in and fed them but as repayment now keep them apart or in very small group so they can't get ideas of how strong they are in numbers.

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In fact, a very good offer. There is one blogger on the Russian segment of YouTube - Ammunition time, who made a video about the possible existence of dragon troops in reality. Of course, the final conclusion was a clear answer "no", because it is too expensive even for medieval feudal lords to feed one dragon carcass that can rise into the air in armor and with a rider on its back. In the series, we have a whole horde of only relatively adult dragons, which, accordingly, should eat tons, causing huge damage to the ecosystem surrounding itself. Therefore, as a lead-up to the emergence of a rational division of food by appointing a viceroy, the idea of a food crisis is more than suitable.
Thanks.