Taming is an action that can be used to Turn a feral Unit to your side, even if you cannot normally produce that Unit type.
A Unit can also be tamed without Magic Item or Artifacts, as seen by Vinny taming a bat in TBFGK Epilogue 16 through unknown means. In IPTSF Text 45, Jillian tamed a feral Yellow Dwagon (which she later named Crapsack) by giving the it several High Elves to eat. Duke Forecastle tamed a double eagle by feeding it fish in part 8 of his story.
Some units cannot be fully tamed, but it is possible to make them friendly to a unit that feeds them.
Real World References
Tame appears to be similar to the real world action of taming or domesticating wild animals.