Black mamba for example:
The venom of the black mamba is highly toxic. Based on the Median lethal dose (LD50) values in mice, the black mamba LD50 is 0.28/0.32 mg/kg subcutaneous.[8][9][10] Although there are other venomous snakes that exhibit higher LD50 toxicity scores, its venom is one of the most rapid-acting. In cases of severe envenomation, it is capable of killing an adult human in as little as 20 minutes. Two such cases have been documented in the medical literature. In one such case, an adult male was bitten on his right arm, just above the wrist by a black mamba which was estimated to be approximately 2.5 m (8.2 ft) in length. The victim began to show signs of prominent neurotoxicity within minutes. At ten minutes post-envenomation, respiratory paralysis set in and 20 minutes post-envenomation the victim showed no signs of life and was deceased.