What is the most common animal organ to be transplanted?

Answer 1

Animal to human transplantation (or "xenotransplantation") is nowhere near as common as some people seem to believe. Generally animal organs are not used since animal organs are not similar to human organs and do not maintain function within the human body.

In exceptionally rare circumstances they are used to try and bridge a gap of a few days, until a human organ becomes available. They are never used as a long term solution (because they do not work long term). In these cases, the organ is best "attached" outside the body of the recipient to avoid unnecessary side-effects from the non-human donor.

Whether they should be used or not is practically irrelevant, since they cannot be used as a long term solution.

Sometimes heart valves from pigs and cows are used in human transplantation, however these do not count as "organs", even if they do occur fairly frequently.