Forretess learns Poison Spikes at lv.1; you must give a Heart Scale to the man in Driftvile City.
Or, there are a number of options for breeding, one I like is:
Breed a male Skorupi with Toxic Spikes with a Female Forretress to get a baby Pineco with Toxic Spikes.
Female + Male = Baby Pineco with Toxic Spikes
Hope this helps! •^-^•