Caatinga Horned Frog (Ceratophrys joazeirensis) metamorph, family Ceratophryidae, endemic to Brazil

Below is an excerpt from Gerald Durrell's 'The Drunken Forest', 1956 (hence some of the language is rather dated), which I think makes the point more eloquently than I ever could:

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There is found in certain parts of South America a toad which must be one of the most bizarre looking of the batrachians. It is called the horned toad, and, as toads as a general rule are easy enough to keep, I thought the horned toad would be simple. For some reason, I had an overwhelming desire to obtain some of these toads while we were in the Chaco. I knew that they were found there, that the local name for them was escuerzo, but there, so to speak, the matter ended. It is one of the strange things about collecting that a creature you are most anxious to capture, no matter how common it was before, immediately becomes nonexistent as soon as you ask about it. So it was with the horned toad; I showed everyone pictures of it, I offered fabulous rewards for its capture, and I nearly drove Jacquie and Rafael mad by forcing them out of bed at two o'clock in the morning to investigate marshes with me in the hope of catching some, but with no success. If I had known the trouble that the horned toads were to cause me, I would not have made such efforts to try to obtain them.

One lunch-time I found a battered tin can, the mouth plugged with leaves, waiting for me on the veranda. Paula could give me no more information than that it was a bicho, which was fairly obvious, and that it had been brought by an elderly Indian. I removed the plug of leaves circumspectly with a stick. Peering into the rusty inside, I saw, to my surprise, a gigantic horned toad squatting placidly on the backs of two smaller ones.

'What is it?' inquired Jacquie, who had kept a discreet distance with Paula.

'Horned toads … three beauties,' I answered delightedly.

I tipped the tin over, and out spilled the toads on to the veranda in a tangled heap. Paula let out a whoop and disappeared into the house; from behind the safety of a window she looked out, palpitating.

'Señor, señor, look out,' she wailed; 'es un bicho muy malo, señor, muy venenoso.'

'Rubbish," I said. 'No es venenoso … no es yarará … es escuerzo, bicho muy lindo.'

'Madre de Dios', said Paula, rolling her eyes to Heaven at the idea of a horned toad being called beautiful.

'Are they poisonous?' inquired Jacquie.

'No, of course not… they just look as though they ought to be, that's all.'

By now the toads had sorted themselves out, and the largest was squatting there, regarding us with an angry eye. He was about the circumference of a saucer, and three-quarters of his bulk seemed to consist of head. He had short, thick legs, a paunchy body, and this enormous head in which were set two large eyes filigreed with a pattern of gold and silver. Above each of these the skin was raised into an isosceles triangle, like the horns of a baby goat. His mouth was incredible, for it was so large it almost appeared to split him in two. The toad, with his rubber lips, horned head, and sulkily drooping mouth, managed to achieve an expression that was a combination of extreme malevolence with the arrogant bearing of an obese monarch. His whole air of evil was enhanced by the fact that he was a pale mustard-yellow in colour, covered with rust-red and sage-green patches that looked as though someone, lacking in artistic and geographical knowledge, had tried to draw a map of the world all over him.

While Paula was evoking the aid of the saints and assuring Jacquie that she would be a widow within half an hour, I bent down to get a closer look at our protégé. Immediately he gulped convulsively, blew himself up to twice his previous size, and then exhaled the air in a series of indignant wheezing screams, at the same time taking little jumps towards me and snapping his great mouth. This was a most effective and startling display, for the inside of his lips was a bright primrose-yellow.

At the sound of the toad's war-cry, Paula clasped her hands and rocked to and fro in the window. I thought this would be a most suitable opportunity to teach her some elementary natural history and, at the same time, enhance my prestige. I picked up the toad, who kicked violently and wheezed asthmatically, and approached the window where Paula was posturing like an outsize puppet.

'Bueno, Paula, mira… no es venenoso… nada, nada,' I said.

As the toad opened his colossal mouth for another bagpipe-like cry, I stuffed my thumb into it. The creature was so surprised that his mouth remained open for a second, and I smiled soothingly at Paula, who appeared to be on the verge of a swoon.

'No es venenoso,' I repeated. 'No es …'

At that precise moment the toad recovered from his surprise and snapped his mouth shut. My first impression was that someone had amputated the entire first joint of my thumb with an extremely blunt hatchet. With an effort, I stifled the cry of agony that rose to my lips. Paula was regarding me pop-eyed, and, for the first time, without a sound. I gave a lop-sided sneer, which I hoped she would mistake for a debonair grin, while the toad amused himself by clenching his jaws as hard as he could at half-second intervals, so that the effect was as if my thumb was lying in the path of an extremely long goods train with more than the normal complement of wheels.

'Santa Maria,' said Paula, 'qué extraordinario… no tiene veneno, señor?'

'No, nada de veneno,' I said hoarsely, still wearing my fixed smile.

'What's the matter?' asked Jacquie curiously.

'For Heaven's sake, get the woman away. This damn thing's nearly taken my thumb off.'

Jacquie hastily distracted Paula's attention with a discreet inquiry about the lunch, and she floated off to the kitchen, still ejaculating 'extraordinario' at intervals. When she had vanished, we turned our attention to saving the remnants of my thumb. This was not so easy, for the toad had an immensely powerful grip but very fragile jaw-bones, and all our attempts at prising open his mouth with a stick caused them to bend alarmingly. Then every time we removed the stick, he would give my thumb a triumphant squeeze. At last, in desperation, I laid my hand and the toad down on the concrete, hoping that this would persuade him to let go, but he just squatted there like a nightmare bulldog, and glared up at me with defiance.

'Perhaps it's the wrong sort of place,' suggested Jacquie helpfully.

'Well, what do you expect me to do?' I inquired irritably. 'Go and sit in a swamp with him?'

'No, but if you stuck your hand into that hibiscus bush, he might feel he could escape if he let go.'

'If he won't let go here, I don't see that crawling about in an hibiscus bush is going to help.'

'Have it your own way. What are you going to do, spend the rest of your life wearing a horned toad on your thumb?'

Eventually I saw that the only alternative to the hibiscus-bush experiment was to risk damaging the toad's mouth in prising it open, so I crawled into the shade of the bush and plunged my hand into the thickest tangle of the undergrowth.

Immediately the toad sprang backwards, at the same time spitting out my thumb with every indication of disgust. I recaptured and put him back in the tin, without much opposition beyond a few half-hearted wheezes. My thumb had a scarlet line round it where his jaws had clamped together, and within an hour an ugly bruise had spread across the nail. It was three days before I could use my thumb without pain, and a month before the bruise faded.

It was the last time I attempted to demonstrate to the inhabitants of the Chaco the harmlessness of the horned toad.