This heavy egg hums with electricity.
Spoiler
A young thunder rat hatches covered in what looks like soft, fluffy fur. Make the mistake of trying to pet it, though, and you'll find them pricklier than a hedgehog; like rekels, the fur of thunder rats is made of sharp metal. As they age, their fur will gradually rust and fall out, replaced with shiny metal scales like those found on the rats' tails. Ancient thunder rats were said to look more lizard than rodent, though the process takes many years and no one has seen a fully hairless thunder rat in over a century.
Spoiler
No one is entirely sure why thunder rats have the ability to produce and control lightning, but some suspect it developed from the need to find metal deposits. Thunder rats exclusively eat metal, and some of the best ways to find it is through its magnetism and conductivity. As the rats live in family groups and go through metal quickly, being able to search for new deposits from long distances may have been advantageous. A rat that could produce and control lightning from a great distance away could find more and better deposits. Regardless, the rats only seem to produce excessive amounts of electricity when travelling or at the command of their magi companion, their fur and scales sufficient to deter most predators at short range. If an over-eager direwolf or dragon continues to pester them, a few warning sparks arcing across their fur is usually enough to send any would-be predator running away.
Spoiler
Thundrer rats are impressive beasts, once used as the primary companions of dark wizards for their ability to conjure and dispel powerful electrical storms. Just one thunder rat could turn the tide of a battle, or destroy an entire village. Their magic can be counteracted by other creatures with strong weather-controlling powers, but back when the Keep was young, such creatures were rare to find as companions. As a result, thunder rats are highly feared, even today. It was long believed that in addition to creating storms, the rats could fly and replicate themselves, and that they emitted toxic breath like otrovian amagnae. Villagers all around the countryside used to create intricate traps for them, and travel long distances just to lay these traps wherever it was said thunder rats had been sighted, for fear they would spread otherwise. Over the years, the rats became shyer and more reclusive, to the point where many magi thought they had gone extinct until only a few decades ago. Since then, the reputation of thunder rats has improved, as magi studying them have found the creatures to be intelligent, gentle, and highly affectionate, even if their powers remain highly dangerous.
