Toads were amphibious creatures native to Earth. The natterjack toad was one of the endangered species that Harrison Chase cited Richard Dunbar as wanting to save. (TV: The Seeds of Doom)
When the Fourth Doctor was accused of being the Evil One in the myths of the Sevateem, he threatened to turn them all into toads. (TV: The Face of Evil)
Bernice Summerfield went to a boarding school on Mal Oreille as a child, where she discovered that the matron of the school was in fact a toad feeding on the students. (PROSE: Biology Lesson on Mal Oreille)
Torm had a humanoid species called "Toad-men". (COMIC: The Body Snatcher)
Due to misinformation from Mr Smith, Sky Smith was under the impression that the dish "toad in the hole" actually had toad in it. (TV: The Curse of Clyde Langer)
References
The word "toad" was used as an insult in numerous occasions.
Firstly, Wessex noblemen, including Irongron, frequently called Linx a toad or "toad face" during the Sontaran's stay on Earth, comparing his skin to that of the Earth species. (TV: The Time Warrior) Another Sontaran, Field Major Styx, was similarly compared to a toad and was nicknamed "the Toad-God". (COMIC: The Gods Walk Among Us)
Ace also called Shou Yuing a "toad face". (TV: Battlefield)
Jeremy Baines once sneeringly called Tim Latimer "that little toad". (TV: Human Nature)