I heard an explanation about pigs that they were not included in Noah's ark. They were in fact from worms. One ate the other and kept growing.
Is there any authentic source for this?
Maybe you heard of a story reported in "Legends of Old Testament Characters, from the Talmud and other sources" by a Rev. S. Baring-Gould, chapter 14
There left the ark two sorts of animals which had not entered it the pig and the cat. These animals did not exist before the Deluge, and God created them in the ark because it was full of filth and human excrements, which caused a great stench. The persons in the ark, not being able to endure any longer the smell, complained to Noah. Then Noah passed his hand down the back of the elephant, and it evacuated the pig. The pig ate all the dung which was in the ark, and the stench was no more.
However this legend doesn't appear to come from the Talmud or midrashim and is either quoted by Baring-Gould from the Koran or some Oriental traditions.