Heck, even a mammal - the solenodon from Cuba - has toxic saliva and a grooved tooth to help ensure that some of that toxin gets into its meal. The solenodon is just about extinct, due to humans and their pets moving there, but it's easy to see how just some small changes could lead to better poison-delivery efficiency: say, a salivary duct with an outlet right above the tooth. Or a groove in a tooth getting a roof and becoming a channel in a tooth.
Of course, they may not truly be poison, and just ugly things to death:
solenodon - Google Search