What the ideal dangerous creature to keep in my moat?

Drain the moat and import a colony of Honey Badgers…because honey badger don’t give a shit. Of course, getting out of your castle might be tricky…

Orbital nuclear weapons are not a practicable defense for a single building.

No hippos. Have you ever seen a hippo take a shit? Enormous volume and exceedingly foul smell. Your property would quickly become uninhabitable.

Sharks, with or without lasers. Or no animals and just fill it with sulfuric acid or boiling tar.

Coincidentally, I am writing a story about a castle with this very problem. Moat starts out with a sea serpent (it’s a fantasy) but the sea serpent leaves at the story’s end. I hadn’t thought of getting a replacement. Keep the ideas coming!

You want the Moat Monster from Blue Moon Rising. It’s monstrous and nasty and that’s all you need to know about it.

I’d fill it with bilharzia and Guinea worms. Might throw in a couple of electric eels too, for good measure.

actually from what ive learned most moats were treated as open air septic tanks …all human waste runoff went directly in the moats and it got to the point that entering the castle could get unpleasant …

Anse Hatfield (of the Hatfield/McCoy feud) kept a bear on a chain for security purposes. Andrew S. Thomas, the son of a friend of his, wrote a memoir about meeting him (among other things) called Wait Til I Winch in the Bear. https://www.amazon.com/Wait-Til-I-Winch-Bear/dp/0934750815

David Spade.
Bye bye now.

Fill the moat with hyperactive 3rd graders.

I think it has to be something that terrifies most people so badly they won’t go into the moat in the first place. That eliminates all types of flesh-eating bacteria (can’t see them), hippos (mostly they just lay there, partially submerged) and pretty much all fish smaller than killer sharks.

Armed, trained killer apes might work, but as we all know, once you give the apes guns, they rise up against us!

That narrows it down to deadly snakes and large mammals. Personally, I like wolves. They do a great snarl, they roam in packs, and their howling will scare the enemy before they ever get to the moat.

Hey! I was planning on getting Riff to keep the rabble out!

Although the rapper of the same name could work too.

Great minds think alike ----- and ours too sometimes.

Cassowaries.

I could be wrong but I don’t think most people would just happily jump into the moat and paddle around with the hippos.

Mimes. They’re not intrinsically dangerous, but most people hate them and will turn & run in the opposite direction when they see one.

But they reproduce so slowly and with being in crowded conditions in a moat disease and hunger are bound to take their toll; that and all those overlapping invisible boxes. No ----- I say they are too valuable a resource and a mime would be a terrible thing to waste on a project like this.
Ya know ---- I’m almost ashamed of myself.

I feel a need to remind you that the reaction to such strong stimulus is referred to as a fight or flight response. The only reason they would run from a moat full of mimes is so that they can go home and get their flamethrowers.

I’m ashamed of myself for laughing at your terrible pun.

A flaming moat o’ mimes would be a lovely sight indeed.

With a keen sense of irony, I wonder how many people would mime roasting marshmallows over the flames? Mime s’mores anyone?