As moddable as DF might be, I have zero experience with dem raws, and those creatures that I would truly enjoy to see in-game, are out of my possibilties, since they are...Well, not really generic.
Swarmling - "A mischievous creature that releases spores growing into more of its kind at an alarming rate."
Basically, a zerg-rush. Weak individually, but can get troublesome due to sheer numbers in appears in.
Powerdragon - "A gigantic reptilian creature. It is mechanical and can breathe fire. Cold and emotionless, they were built with ancient technology and can fly."
Basically, a dragon made out of steel/iron/adamantine that can fly (supposed to have jet engines, but that makes no difference from wings, I guess) and breaths fire. Doesn't feel pain, anger, any other emotions; Can't die of old age.
They were probably made in one form or another, but hey - they SHOULDN'T (although I don't know much about raws) be TOO hard to make, right...
RIGHT?
The Powerdragon would be fairly easy. Changing the material's the body is made out of is straight forward, you could just copy the current dragon (or cave dragon) raws and change the creatures mats for skin and bone into steel/adamantine/whatever.
There are several ways to go about doing this. If you want a (semi)tutorial on it, check out
Shaostouls's Modding Guide. Look in the section marked "Basic Body Modification".
Side note: You don't need wings to have a creature fly, you can assign whatever body part you want to make it fly, legs for example if your jet engines come out of its legs. Or the tail, or maybe lower body, if its jet engines come out of some other orifice. As it stands now, I'm pretty sure the standard DF dragons don't fly or have wings (cave dragons have wings but don't fly). So a flying powerdragon would require some new additions in the body plan department if you started from a copy of the dragon raws, and in either case need to have the [FLIER] tag added into the raws to allow them to fly.
The simplest way to get a winged and flying dragon from the dragon raws is to start out by adding wings to it. Scroll down to the section that looks like:
[BODY:QUADRUPED:TAIL:2EYES:NOSE:2LUNGS: and so on...
and add in the token 2WINGS into the list. Easiest to do it in the front i feel. So our changed body plan will look like this:
[BODY:QUADRUPED:
2WINGS:TAIL:2EYES:NOSE:2LUNGS: and so on...
You also need to assign the [FLIER] tag in the creature raws to have it actually fly. Somewhere earlier up in the raws, like where the [BIOME:ANY_LAND] token is located, might be suitable as it is easy to spot.
The [NO_PAIN] and [NO_FEAR] tokens handle pain and fear respectively. I think that as long as there is no [MAXAGE] defined for the creature (there isn't for either dragon), then the creature can't die from old age. Someone with more knowledge than me might need to pipe up on that one.
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As for the swarmling, the breeding behavior you described for a wild animal (or enemy creature) is not practically moddable. However, just an early zerg rush type of enemy creature is more than possible, it is easy and FUN!