Wild pigs have great courage and strength in a group. The reason for this is because the group is not specific. You are referring to any group of wild pigs in general. If you use "the", then it must be specific and relate to a particular noun. For example, "in a group of wild pigs, the largest pig shows the most strength and courage." In this case, you are singling out a particular pig (the largest pig) out of a group of pigs, hence the use of "the".

'In a group' We are talking about all wild pigs, who could form any group. If we were talking about a certain number of pigs (those living in a given area) who migrate to just one group, then we could say: The wild pigs in this area have great and strength in the group.