Brazilian wandering spiders, and this is the bluntest page on the site.
The venom is among the most medically significant of any spider, envenomations are a serious clinical event, and antivenom exists in Brazil and largely nowhere else. Its powerful neurotoxic venom can also trigger extreme muscle spasms, shock, and—famously in men—a painful, long-lasting erection known as priapism.
They are nomadic, building no web, hunting on foot at night, and their defensive display involves rearing up with the front legs raised, which is a warning and should be treated as one.
They are also fast, large, and adept at leaving an enclosure. Availability is sporadic, and legality varies. There is no version of this animal that is a reasonable purchase for someone who is not deeply experienced, has not arranged a plan for a bite, and does not have a genuinely secure enclosure.