Hallucinogens could be used by being smoked or possibly eaten.
Although it is impossible to know for sure, the species Amanita muscaria was likely the first mushroom used as a hallucinogen.
LSD was first discovered and used by Albert Hofmann.
Many different ones. The native American church is known for using peyote which contains mescaline. In south America they use mushrooms, DMT (ayahuasca), and cacti (mescaline). It really just depends...
PCP is sometimes considered an hallucinogen because it has some of the same effects. However, it does not fit easily into any one drug