A serpent

You've heard legends of the lamia, though there's no proof they ever really existed. The image of Doe slithering up to you and cuddling for warmth is delightful and you decide there's no need for your new mount to be particularly fast. Snakes can manage reasonable speeds and they're very quiet. You're sure she won't have any trouble keeping up with an army on the march, and in combat surprise is worth a great deal. An enemy mage can't counter your attack if they don't even know you're there.

Combining a woman and a snake is going to be difficult, but that will also show your peers that you aren't to be trifled with. Creating enough snake flesh and linking up the nerves and skeleton will be somewhat hard, but manageable. Dealing with the warm-blooded versus cold-blooded issue will be the big challenge. You probably want a little bit of a mix of the best of both worlds - it would suck to do all this work only to end up with a lamia who can't do anything without curling up next to a roaring fire for hours. Or one that constantly sweats and overheats.

Finally, you get back to the city. You're eager to return to your home and consult some texts. Get started on planning this lamia idea out, feel out Doe and see how she's behaving.

You almost skip the victory parade, but it's a good thing you didn't. Something strange and fortuitous happens - many of the knights and horse archers start swaying and swerving violently. Soon it becomes apparent that their mounts are the issue. Some collapse, some careen across the street, almost as though drunk. You get involved quickly, placing the horses in a deep sleep to keep them from harming themselves, their riders or the crowds.

What happened comes to light as soon as you have a chance to examine one of the affected animals. Blue nail poison - rare, expensive and dangerous in even small quantities. It seems one or more of the captives managed to keep some of it hidden all this time. They probably wanted to put it in the victory punch, but instead they mixed it in with the feed for the animals. Although you manage to save many of the horses it's still a disaster for the cavalry and a real blow to the army. It doesn't help that it makes a lot of important people look like idiots.

You use a spell to keep the animals that aren't too far gone in a half-living state. You excuse it as triage, to give you time to properly cleanse the systems of the horses that got smaller doses. The truth is that the animals that are already back on their hooves are the ones that will survive. Blue nail rots the brain, rather irreversibly. With your head already thinking in the direction of centaurs and such the sight of tons of otherwise useless horseflesh gives you an idea.

You have the excuse, served up to you on a silver platter. All you need now is the permission.

Later, at an audience with the king...

"None of them can be saved?" asks one of the king's top councilors.

"I'm afraid not. There were a few that might have, but the poison did its work too quickly. There's a reason they use that stuff on nobles and sometimes even mages." That sends an uncomfortable murmur around the table. A well timed reminder of how bad things could have been. Then you wait a while for other people to voice opinions and bring up more bad news. You aren't disappointed.

"...can't possibly replace them within the next year..."

"...should have just killed them all..."

"...not as many spoils as we hoped from the siege..."

"...grain shortages..."

"...our enemies will soon know of this..."

When you deem the room receptive enough you wait for the appropriate gap in the conversation and then announce, "I have an idea - I've always been a 'two birds with one stone' type..." Sure enough a ray of hope at this moment, even if it's coming from a mage most people worry about, is welcome.

"We need new mounts and new beasts of burden for the army? We've got a bunch of captives we have to punish to send a message. Then there are a pile of perfectly good horse bodies out there, and all they need are some new minds in charge of them. An army with centaurs for mounts would be striking if nothing else."

There's a bunch of wild questions, muttering, even a prayer - though you can't tell if it was an apology to the gods for what's being suggested or in thanks for a solution being found so quickly.

The king motions for quiet and asks you, "But can it actually be done? That seems like quite the feat."

"I've been thinking about centaurs and related creatures for a while, and yes, I think so." Let them all assume 'a while' means months or years, not the last few days while coming home.

One of the older councilors asks, very respectfully, "Are not centaurs known to be a prideful race that refuse to be ridden?"

"Real centaurs, yes," you reply, "but in this case... hmmm, perhaps some of them will become prideful of their new coats and bodies in time. But the act of being changed should be a shock to the system. It may make bonding with their new master easier." That last bit gets the interest of two of the cavalry officers. "And if they know the narrowly dodged alternative was execution...."

"They would be fast, it would be easy for them to escape," comments one noble.

"True," says another, "but they would be hard to miss. They would have to escape the whole country rather than to the nearest village receptive to their plight."

"Excuse me," asks the polite, older councilor. Sir Larkan, one of the sharpest among them. "Lord John, what would you desire in all this?"

"Certain warrants against interference from the church, for a start," you tell him. Those are always handy. "It would be best for this to have been the king's idea, if your majesty does not object. Fewer people will complain if it's portrayed as a pure act of crime and punishment."

"With yourself as the mere executor, as it were?" He sips his drink. "Something to keep yourself free from much of the vitriol of the church. So it shall be. They already think of me as a tyrant, I may as well ensure they fear me."

"I also have some special plans for at least one captive in particular and those plans may require some aid in time. But all that can wait. Creating centaurs is my first priority - no sense doing all that fighting only to risk things by appearing weak to our hungry neighbors."