I saw on the wiki that it does not appear currently that there are any in-game impacts of a lion being born tailless other than the visual change. I thought it would be cool if flavor text could be added when the lion's den is visited, since in the real world lion's tails indicate their mood and help them with balance. Some example ideas:

Cubs:
When trying to climb a tree in your territory today, [name] fell down due to being unable to balance properly, causing some of the other little ones to laugh at him/her. He/she just huffed and walked away.

[name]'s mother seems to be a little confused as to how to groom her child, since she keeps on expecting something where there is just smoothness.

Adolescents:
[name] padded up to you to tell you about the events of the hunt/patrol he/she tagged along on earlier. Apparently, lions can sometimes get confused and bat at their own tail! So weird. You've definitely never done that...

Adults:
When you walked up to [name], sitting in an isolated part of your territory, he/she hissed at you and made a swipe at your face. Looks like he/she didn't feel like talking to anyone. How were you supposed to know?