Men were immune to eels.
Toothless had thought his man evil the first time he'd brought him an eel, coughing it up out of his large wicker gullet. He'd thought he had come to finish Toothless off, to sicken him with eel venom and to cut his throat open while he suffered.
Though he'd been at his weakest and his most desperate, he refused to die and had been proud to foil his man's wicked scheming. He'd been weary as he had tried to sneak up on Toothless afterwards.
He'd thought his man was sick the second time after he'd brought home an eel, holding it up like he was proud of his catch. He was so diseased he'd not even had the dignity to spit it out of one of his many wickers.
Toothless had been on guard afterwards, ready to watch over his man during his final moments and to put him down if necessary. But… his man was fine.
He behaved as he normally did, crooning at Toothless soothingly. He seemed to be in control of his senses. Well, as many senses as a man was allowed to have.
He'd been fine long after that, too.
He confided in Hookfang, who had been alert with his own man after hearing the news. Meatlug overheard and had been frantic the next day, reporting that her man had done the same in the night while she had been unable to protect him.
The two-headed one was not worried. Their two men had been eating eels for many, many days and they had been fine. It was just what they did.
He and Hookfang and Meatlug had been cross to hear that at first, especially Meatlug. She insisted that it was important to care for your men, as they were delicate and needed a lot of caring, like an egg. What if they'd been broken?
Toothless wasn't sure if he agreed with her completely. Hookfang rejected the idea, though he found some merit in it. He said her man was just high maintenance.
Hookfang suggested that that was why the two-headed men were so odd. They hadn't had enough minding. The two-headed one insisted they were just fine and that their men were self-sufficient enough to not need any.
They had been circling and snapping at each other rebelliously, feuding for the rest of the day and the one afterwards. They'd put many scratches into the large metal-stone basin.
But... Well, the more Toothless thought about it, the more the two-headed one made sense. About the Eels. Eels made insanity and men were already insane, so they probably had no effect on each other. After all, what sane animal would eat an eel?