I suspect you're looking for arsenic as your answer, as this element is effectively toxic to mammals, including mice, rats, etc. However, it'd be the wrong answer for a few reasons.
The first reason is that arsenic is no longer used as a pesticide pretty much anywhere in the world, due to its toxicity to humans, and its tendency to remain in the environment.
The other reason is that, in sufficient dosage, a huge number of components of the Periodic Table -- elements -- are toxic to humans, and some are hyper-toxic. Polonium, for instance, (Po 210) has an LD50 (Lethal dose in 50% of the test population) of perhaps 0.1-0.3 GBq, making it far more toxic than arsenic.
iodine
Arsenic
- alloying element for bronze - poison used by murderers
chlorine
arsenic
Several. Arsenic was once used as a rat poison and also has use in medicine (bone marrow disorders as an example. Thallium was once used as a rat poison and was once used to treat Syphillis Barium used to be used in rat poison and is used in medicine (barium meal) for some types of scan. Mostly these days however it is a compound that applies to both rather than an element - that being Warfarin.
Arsenic is a commonly used element in rat poison due to its toxicity to rats and other rodents. It interferes with their ability to produce energy at a cellular level, leading to their death.
The element used in poison ivy lotions is typically calamine, which helps to soothe skin irritations caused by exposure to poison ivy. Calamine is a mixture of zinc oxide and ferric oxide that has anti-inflammatory properties and can help relieve itching and redness.
bromide
Arsenic is used in rat poison and is in the title "Arsenic and Old Lace."
an element that i think is a very old poison is mercury Antimony Arsenic Bismuth Mercury Copper Cadmium and Lead are all poisonous and used in old paint
an element that was used was chlorinethere are some others such as tear and mustard gas