Of the three approved animals permitted to students as pets at Hogwarts, the toad is, and has been for many years now, by far the least popular. Centuries ago, in blood-thirstier times, when young witches and wizards were expected to personally pop out the newt eyes they were using in potions, they routinely brought boxes of toads to school for use in potions and in other charms. Over time, as the Ministry of Magic introduced legislation regarding animal cruelty (sub-sections 13-29 inclusive relate to potion ingredients and their production) such practices were gradually outlawed. The toad, never much appreciated for its own personal appeal, gradually appeared (alive) less and less frequently at Hogwarts, unless hopping and swimming wild in the grounds.

By the time Harry arrived at Hogwarts, possession of a pet toad conveyed neither cool nor status; indeed, it was something of an embarrassment. Trevor, Neville’s toad, had nothing to commend him except a propensity for getting lost, and when he finally slipped off to join his brethren in the Hogwarts lake, both owner and pet felt a sense of relief.

J.K. Rowling’s thoughts

The toad has a long association with witchcraft, and was often believed to be a familiar. It occupies a special place in old folk cures, particularly (perhaps on the homeopathic principle of curing like with like) in the curing of warts. In the Dark Ages, a British toad could think itself lucky if it died of natural causes, because it was in constant danger of being boiled, powdered, skinned or tied around a sick human’s neck in a bag.

Crookshanks tries to attack Scabbers in the Diagon Alley petstore