Often cited as the first is the Leave It to Beaver episode "Captain Jack" which aired on October 11th, 1957. However some may not feel that this counts since only a toilet tank is shown, not a toilet bowl. In the episode Wally and Beaver purchase a baby alligator, and attempting to hide it from their parents they decide to keep it in the toilet tank.
The episode was intended to be the debut episode of the series. However problems regarding showing a toilet on television caused it to be postponed while the issue was worked out with the censors. The producers considered other places in the house that could be used instead, but concluded that the toilet tank solution was the only one that worked within the script. They eventually reached an agreement with the censors that as long as only the toilet tank was seen and not the bowl it would be allowed. With this change in place Captain Jack aired as the second episode in the series. While it seems tame by today's standards, it was somewhat daring for 1957.