Despite not being a castle history expert, allow me to shed some light on the subject. However, first the obligatory too long didn't read: the stereotype as films have portrayed it originated most likely from a 19th century politician named Benedetto Croce (Italian in origin). In reality we have no evidence that any self-sustaining castle intentionally contained a crocodile in its moat. Now for the longer explanation.

A moat by function is rather hard to maintain; many castle moats of the medieval and even late 16th century fell into disrepair and as a result it resembled less of a circular water defense pool and more of an algae-filled bug-infested bog surrounding a grand estate. By its very nature moats demand constant draining and cleaning, so the very idea of creating a moat in order to keep a wild and dangerous animal inside of it beckons a whole new plethora of issues. However, even by its function a dry moat is just as good as one filled with water, so many castles in later eras did away with the troubles of filling up their moats and made "dirt pits" to deter enemy forces from assaulting them head-on.

Of course, the equally stereotypical image of enemy soldiers floundering in dirt pits or drowning in moats is rather interesting; as moats themselves were mostly designed with the intention of stopping a favourite staple of siege warfare: tunnel digging. The principle was that a moat would prevent enemy forces tunneling underneath the original walls and simply causing them to collapse from the inside (if that moat had water in it, then so much the better to drown enemy soldiers underground).

As for moat-implanted defenses we don't actually have any records of noble folk or garrisons purposefully placing animals in them to deter enemy forces further (except for one notable exception I'll get onto later). We do however, have archaeological evidence that some castle moats had stinging bushes planted in them (dry moats obviously), though whether these were actually ordered by the defenders or simply a byproduct of natural processes in that moat. Some moats did have fishes in them (no piranhas or anything deadly mind you), but these were viewed as status symbols rather than actual defensive mechanisms.

Now onto that exception I mentioned: Český Krumlov Castle in the Czech Republic has a bear moat (or more accurately, a bear pit). The first recorded origins of this intriguing feature date back to the 1600s, when Bohemian Count Wilhlem von Rosenburg bought a couple of bears from Transylvania and placed them in the castle as a deterrence to enemy forces (it's also believed, though no sources exist, that prisoners were simply 'thrown to the bears' as punishment). For those curious, that tradition still exists to this day: there is always at least one bear in that castle (though nowadays for ceremonial purposes rather than actual defense obviously).

So what about that stereotype? Quite simply: no concrete records exist to prove that a crocodile was put inside a castle moat. We do however, possess rumours of a crocodile moat in the Castel Nuovo of Naples. Those rumours however date from the Italian historian Benedetto Croce, writing in the 19th and 20th century (so what they're worth is up to you). In his Neapolitan Stories and Legends, he writes of the Castel Nuovo under the reign of Joanna II of the House of Anjou:

In that castle, there was a moat under the level of the sea, dark, humid, where the prisoners, who they want to more strictly castigate, were usually put. When, all of a sudden, they started to notice with astonishment that, from there, the prisoners disappeared. Did they escape? How? Put a tighter surveillance and a new guest inside there, one day they saw, unexpected and terrifying scene, from a hole hidden in the moat, a monster, a crocodile entering and, with its jaws, it grasped for the legs the prisoner, and dragged him to the sea to eat him.

Apparently popular myths of the mid 15th century suggest that Joanna somehow managed to smuggle an African crocodile across the Mediterranean into her castle, where it became an "executioner of justice" to both prisoners and (somewhat cynically), her former lovers. The story even posits that she had a well designed specifically for the purpose of "disposing of her former interests". How credible these tales are we're not exactly certain, but the lack of any other legends or stories makes it clear that this (if it was a. thing) was a one-off moat.

So to answer both your questions: a flimsy tale of an Italian moat infested with a single crocodile in the 1400s gave rise to an decades-long film cartoon stereotype of castle moats being infested with crocodiles. Given that they're the ideal mix of hostile yet docile and perfectly suit the structure of a moat, it's not surprising they were chosen over other deadly swamp creatures. Beyond that tale it never actually happened, both due to the sheer impracticability of maintaining a live crocodile in European climates and also the rather punitive addition of a crocodile (or any other animal for that matter) to a moat's already impressive defensive purpose. Hope this helps answer your question!

Hi,

While you wait for an answer, you may be interested in this previous question:

• Is there any evidence of someone having a crocodile/alligator in the moat of a castle? Answered by /u/sunagainstgold.

I couldn't find a good previous answer regarding the origin of the trope, so hopefully someone can chime in on that!

This was also answered in brief before here, that it may have come from Sri Lanka or the Tower of London. .

Additionally, the reason you may have a cultural memory of this is due to its use in animation. There are scenes with crocodiles/reptiles in moats in Disney movies like The Sword in the Stone, Warner’s shorts, and I believe Ren and Stimpy uses the gag. I couldn’t locate a film historian’s view of it quickly, but unfortunately in my experience, most origins of gags in animation (for example the common trope of cross dressing to fool the antagonist, most famously by Bugs Bunny) are chalked up to “we just thought it was funny.” While there are normally deeper cultural meanings behind the gags from a sociological or psychological point of view, it’s harder to get a satisfying reason behind the bits. I have a historiography somewhere to provide more sources if necessary.