I used a mutagen just out of curiosity.  It changed a "gracefulness" recessive to a "normal" recessive.  :(

I haved only used them on one and only on a pet I was considering selling. He had Weak Muscles to start with so I was meh about him.

First time I got Clumsy paws. Ew.

Second - thick skin

Third - Solid horn

Fourth - Smart

1 red and 3 very rare greens ... I can't complain though I don't think I'll mess with him anymore, might lose some of those lovely greens lol.

Congrats, Shilar! O: 

Finally found a mutagen in one of the shops and actually had enough money to buy it. I have this unicorn with pretty lousy stats, so I thought, "What have I got to lose? She's already got dominant poor hearing, fragile bones, and dystrophy, so it can only get better from here, right?"

Now she is also recessive for faint-hearted instead of normal. 5000 silver down the drain.

i have all but killed my poor bruce -http://beastkeeper.com/pet/4522

so much red on him now ... though i can mostly say he can not get too much worse at this point ... today for exploring he is lvl 35 he now losses ALL his island battles he even loses to lvl 30s now ... i officaly cant level him up without the use of crafting exp/(if i had a riding ring lvl 3-5)

am i giving up though NOPE ... wasting money 5k at a time to try and fix him now to where i can at least start leveling him again. once he gets there i am swearing off of using mutagens ... way too addicting with too much risk @..@

So far I've tried 3 mutagens out on one pet and the pet has just gotten worse.

Changed the color once, and then took out one of his good stats and just now, changed his lionheart to faint-hearted. D: Fuuuuu-- 80-something courage down to 27.

So, mutagens... useless? I think so.

From now on I think I'm just gonna auction them off unless they decide to buff mutagens to actually give GOOD genes sometime!

EDIT: I made a suggestion thread ranting about how they need to buff mutagens. http://beastkeeper.com/thread/1529 Please go there and support!

Got a thick skin. C:

It seems like the best course of action would be to use a few mutagens on a pet that you are indifferent about, that has mostly normal genes. That way there are no good stats to lose, and if you end up changing one or two to stat boosts, you win, and if you change to bad stats, there's always the State Portal Service. :)

I've only tried mutagens on one pet, my first pet I got during the tutorial, Gabriel. I don't remember exactly what changed, but I know it changed his recessive marking form Normal to Piebald, and I think it also gave him Massive Hooves recessive.

I have a pet I specifically used for Mutagen, since she had really bad genes from the start. I wrote everything down in a journal on on her page in the profile area so I can keep track on how she respence to the mutagens should I even need to look back.

http://beastkeeper.com/pet/11393

Ok, I just used 12 more mutagens on Gabriel. Red means it changed to a bad gene, orange means it changed to normal, green means it changed to a desirable gene, black is color gene related. The interesting thing is that it can also change genes from dominant to recessive and vice versa. Here's what I got:

Normal Horn -> Small Horn (rec)
Normal Muscles -> Dystrophy (rec)
Quickmind -> Normal Wits (rec)
Massive Hooves -> Agile Paws (rec to dom)
Normal Skin -> Magic Mirror (rec)
Dystrophy -> Steel Muscles (rec)
Buckskin -> White (rec to dom) image changed from Chestnut Overo to White
White -> Black (dom to rec) image changed from White to Chestnut Overo
Agile Paws -> Small Hooves (dom to rec)
Piebald -> Leopard (rec)
Small horn -> Normal Horn (rec)
Thin Skin -> Fragrance (dom)

So from starting genes to final genes:

Recessive: lost 2 good (Quickmind & Massive hooves), gained 2 good (Steel muscles and Magic mirror), gained 1 bad (Small hooves), and changed color and marking (buckskin to black, piebald to leopard).
Dominant: Changed a bad gene to a good gene (Thin skin to Fragrance).

This is my project. got from the pound today.

Mutagen Project Unicorn

 so far...

http://beastkeeper.com/pet/75924

I believe I used ten mutagens on Elijah.... I improved his strength with strong muscles, changed his color two or three times, changed a few recessives, and.... made him faint-hearted.  Sigh.  

I was hoping I could fix up his intelligence issue there, but I suppose that just didn't happen.... =P

Well I checked my inventory & discovered I had 3 mutagen, used all 3 on my blue male cockatrice, got 1 bad & 2 OK results;

Poisonous Claws changed to Clumsy paws

Dystrophy changed to Steel muscles.

Normal wits changed to Smart.

Got best pets ever from mutagen, and now I got lines of powerful, impressive offspring who do well for breeding,

Got bet expert, animal instinct, steel  muscles...wish the color changed, I got way to many beautiful blue cockatrice, lmao XD

oh, acid glad and stuff, only bad is faint-hearted and clumsy paws.