There do exist slight differences between various verses of different biblical manuscripts (though none of these have any impact of Christian doctrine).
Scholars have put much effort into determining what the content of the original copies were. This is done by analyzing old fragments and manuscripts of the Bible that have been discovered. Among the oldest are the Dead Sea Scrolls, which include a complete copy of the Book of Isaiah, dating from 356-103 BC. In verse Isaiah 11:6, this copy appears to use the Hebrew word for "wolf" (זְאֵב zeh-abe') and not the word for "lion" (אֲרִי ar-ee').
(Isaiah 11:6 is underlined in green)
The original version of the King James Version, published in 1611, seems to agree and also uses the word "wolf".
The verse Isaiah 65:25, which uses similar imagery, also uses "wolf" in both the Dead Sea Scrolls and the original King James version. On this note, it is amazing how accurate God has maintained copies of the Bible for three and half thousand years!
I don't know when or how the word "lion" become a popular rendering of these verses. Some have suggested that it's a contraction of the entire verse.
The wolf shall dwell with the lamb, and the leopard shall lie down
with the young goat, and the calf and the lion and the fattened calf
together; and a little child shall lead them. (Isaiah 6:11)
he wolf and the lamb shall graze together; the lion shall eat straw
like the ox, and dust shall be the serpent’s food. They shall not hurt
or destroy in all my holy mountain (Isaiah 65:25)
In any case, "The lion shall lay down with the lamb" version of these verses has become well known in our society, as you have noted. However, it important to remember that, as Christians, we are not the followers of any particular preacher, but rather we are followers of Christ (see 1 Corinthians 3), and that "All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work." (2 Timothy 3:16-17)