Here is the verse in question:
1 Chronicles 12:8 And of the Gadites there separated themselves unto David into the hold to the wilderness men of might, and men of war fit for the battle, that could handle shield and buckler, whose faces were like the faces of lions, and were as swift as the roes upon the mountains;
The Gadites were the men of the Israelite tribe of Gad. Their territory was on the eastern side of the Jordan River in the highlands. They were known as a warrior tribe. This may be accounted at least partially to their rough life in the mountains of the east and to the fact that they were a border tribe that was separated from most of Israel by the Jordan River. They had to be ready to fight at a moment's notice.
This verse says that they had faces "like the faces of lions." They did not have the faces of lions. Rather, in some way their faces resembled that of lions. The use of the word "like" is very important. It points to similarity, not equal identity.
Lions are very distinguished animals in the Bible. They were known for their strength (Judges 14:18), their bravery (2 Samuel 17:10), and their fierceness (Job 10:16). Many people in the Bible are compared in one way or another to lions. Judah is "a lion's whelp" (Genesis 49:9). Israel shall "rise up as a great lion" (Numbers 23:24). Jonathan and David are said to be "stronger than lions" (2 Samuel 1:23). The list could be much longer. Benaiah, one of David's mighty men, killed "two lion-like men of Moab" (2 Samuel 23:20). This certainly refers to their strength and fierceness.
The Gadites are compared to lions in another passage. In the blessing of Moses on the different tribes, he blesses Gad with these words: "Blessed be he that enlargeth Gad: he dwelleth as a lion, and teareth the arm with the crown of the head" (Deuteronomy 33:20). The tribe of Gad was compared to a lion.
So, what does the Bible mean when it refers to the Gadites as having faces like the faces of lions? It is describing the Gadites as having faces that were as fierce and bold as that of lions. They were not mutants, only fierce and strong examples of the human race.