BEFORE reading into my suggestion, I apologize in advance if this has already been suggested. (Turns out it has, here's the other post about it http://www.lioden.com/topic.php?id=304429231058 ). I honestly don't know what to make of this mutation or rather even what to call it so I'm sorry if this was already suggested. I did look at Xylax's masterpost of mutations to be sure it wasn't already in the works.

I stumbled upon this article that really surprised me: Mutation here (slightly disturbing but not too bad)

I realize that this had happened to a mountain lion, but I still find it fascinating. This obviously had to have been a birth defect, however I don't think it would end up being a lethal mutation at birth at least. From the photo in the link provided, it's hard to tell how old the mountain lion could be but I'm guessing at least adolescent or older.

In the article, they began talking about how it could have happened: Either a conjoined twin who dies at birth, or a teratoma tumor. I researched more into the teratoma option on wikipedia, and I have discovered that these kinds of tumors aren't really dangerous to fetuses unless there is mass effect or a large amount of blood flow in the tumor (vascular steal). With this information, the mutation could be lethal for newborn cubs. Though if this information was true for the mountain lion in the other article, it should have died early on in it's life but it didn't. So I came to a theory that maybe if this could be a new mutation for the game, there could be maybe 2 stages. I don't think it's possible for a mutation to be both lethal and not, so I don't think it's a good idea to suggest that. I'm still trying to wrap my head around this mutation idea and I may need to put in a little more research into this, but for now I think I'll leave it as this simple little intro and to either gather thoughts from you guys, or to be given reasons why this shouldn't be an addition to the game. Either way, please don't be rude and be respectful in your comments ^-^

Update 1

Regarding whether or not the mutation would be lethal, the mountain lion in the picture obviously had not died due to the cause of the tumor but was killed by hunters. The only question remaining is whether or not the cougar would have ended up dying later on in life with complications of the tumor, or would it have been fine? This leads to my addition to the idea that the option of the mutation being lethal or not is open to being either both (2 different stages) or just one of the two. I feel that in order to make this more simple for the game, we would all have to settle with just one stage (which would be either or). Unless the game is capable of having code written for two stages of this mutation as lethal and non-lethal, we should all decide in the comments whether you support a lethal mutation or non-lethal. So far, I am supporting non-lethal only with the support of the article I had provided of the cougar. There is also evidence that it could be lethal, but I think more research needs to be put in on that on my part. For now, comment what you feel would be a better choice: Both Lethal stage and Non-Lethal Stage, or just one of the two.

Update 2
With time in between studying for finals, I went into more research on this type of mutation. I've read that for animals this type of mutation is known more of an Ovarian teratoma. It's been found in mares, the mountain lion mentioned in this post, and even canines. I've been trying to find accurate proof as to whether or not this mutation would be lethal or not. I've seen that the only time it's harmful is if it really affects blood flow from the heart, and I believe the term is vascular steal unless I'm interpreting this wrong. Another thing to add, the word "teratoma" is a Greek word that translates to "monster" and has the ability of growing teeth or hair. Since we already have an overgrown fur mutation, I would have to say this mutation could be strictly teeth only like the mountain lion. So that narrows this mutation down to one form which is progress in this research. The research as to whether this is lethal or not is still a work in progress. Still, from the article about the mountain lion (not sure if what I'm about to say is completely accurate) but the mountain lion seemed healthy and just fine before it was shot and killed. Though there really isn't enough evidence about the behavior of the mountain lion, that leaves this research about the lethal aspects of this still a work in progress.

Update 3
I apologize upfront about how this update may sound, I'm still getting over from being sick and I may sound like I'm rambling or may not make sense at some parts of this update. I had found some new and interesting things I wanted to share but I was having a hard time understanding it, so I thought maybe writing could somewhat help. I began doing some more research to try and get a better understanding of this whole process. I've discovered that there are 2 stages of ovarian teratoma: Mature and Immature, mature being benign and immature being cancerous. I started thinking that I would never really find a good solid answer as to whether or not this mutation should be lethal or not, until I discovered I was looking at this all wrong. I was researching only into the ovarian teratoma tumor, where I should have been researching more along the lines of a Dermoid cyst. I think this was the proper term for this mutation instead of teratoma, though I could be wrong... I think they are the same thin though. However, when researching the dermoid cyst I was finding more facts and aspects to the tumor that made it seem more helpful in developin this idea. I had found that the dermoid cyst also has the same symptoms such as growing teeth, bones, hair, etc. I was also discovering more useful facts, such as that this could occur anywhere from birth to early adulthood, so if this mutation were to occur in lioden it could happen at any time in a lion's life. Now regarding whether or not this is lethal or not, I want to say that in my honest opinion this should be a nonlethal mutation. Unless it was some super and incredibly rare case, these mutations are not lethal. It was evident in the mountain lion that it was still living and going on about life normally. And it seemed to have been grown up and doing just fine.

So I believe that with all the research I've put into this, my final idea for this mutation should be this: A Mature(Benign) Teratoma Tumor/Dermoid Cyst that is non lethal and can occur at any stage in the lion's life. To make things a bit easier in the art world, I also vote that we keep the appearance the same as the mountain lion. OR maybe, if it's possible, to have the teeth also appear in other places. Though I think it'd be easiest to just keep the appearance teeth growing out of the skull. That is my final proposal, and now in the comments we can really discuss how to finalize this last proposal I have made. If needed, I could do more research but I fell I have put a lot of research into this already and I'm starting to feel like I am repeating myself a lot. Please feel free to continue pitching in your ideas and also critiquing whether or not some parts of this is good or bad. I hope that you guys support this, and of course I understand if you don't.