I can’t help but wonder if somebody else has had this idea before.
Thundercows
These bioengineered animals were designed to circumvent the high-technology ban that Galactic civilization slaps on trade between advanced and primitive planetary cultures. Said ban is interesting, actually: the primitive planetary cultures hate that ban because they can’t buy cool gadgets from advanced civilizations. The advanced planetary cultures also hate that ban because they can’t sell cool gadgets to primitives. And the Galactic government hates the ban because everybody screams at them over it and the paperwork is sometimes almost literally murderous. And yet, nobody can ever manage to get the relevant laws repealed. It’s either weird, or a conspiracy: take your pick.
Anyway, Thundercows appear to be fairly standard Terran cows (bos taurus), and they mostly are, with one small exception: when properly primed, Thundercows excrete black powder. Well, more accurately: they excrete discrete wet nuggets that, when dried and powdered, become black powder. The bacteria that do the transformation typically live in the spinal column of the Thundercows, so people start the process by killing a Thundercow and grinding up its spine to put in the other Thundercow’s feed. The species can breed on its own, but they aren’t very fertile. They also taste horrible.
As you might imagine, Thundercows are very popular where gunpowder is a restricted substance. Quite a few primitive planetary cultures have put their own bans in place when it comes to gunpowder: would-be rebels and usurpers will pay good money for a few Thundercows. That every so often one of those cows explode is usually seen as merely part of the cost of doing business…