the first idea is a mutation where a lion grows a mane over time, for example, a female is born with no mane but later on say at random gets a line of mane like that to an adolescent this could be any age than over time the mane grows into a full mane and doesn't stop at random going from different stages at different times like stage one is some cheek and elbow fluff then the next stage is scarce mane around the neck then a little longer than full mane once adult all at random on a roll at any stage of life but is void before saying 2-3 months of age
this idea will have a stopping point for the mane but it should grow longer than the mane it shows looks on a non-mutant male or female

the next idea is a mane that grows randomly over the lion's body over time likely to occur most in males rare in females causing random clumps of main all over the body and perhaps a really rare full-body mane shortening at the face like fur

I think if we can only have one that the first idea is the best and easiest for developing