How to Unlock Undine Guide
In order to unlock the Undine monster you'll need to raise a Hopper up to at least B rank and then wait for him to dig up the Hot Springs during the winter months (Dec, Jan and Feb) on your Ranch. According to everything I have read online this has a completely random chance of happening, but there's some things you can do to increase your chances though.
First up is don't let your Hopper get any fatigue or stress all Winter. Give him Mint Leaf before winter to make sure he isn't stressed at all and do nothing but Rest him throughout all three months. The second thing I recommend you do is raise a Hopper to B rank and then freeze him, only unfreezing him for the winter months.
This all is assuming you have as much trouble as I did getting Hopper to dig up the Hot Springs. I went through 35 winters and 4 different Hoppers; that doesn't include the winters I went through and reset the game for either. I did at least a dozen winters then reset the game and did the same winters again. I stopped doing that though when I felt like the winters were exact repeats. My Hopper found Flowers and Galoe Nuts for Colt on the same days each time and it felt like the winter's events were already chosen before I even went through the day.
Note: As you can see by the lack of screen shots on this page I was never successful in unlocking the Undine monster. I have no idea what I did wrong or if I was just insanely unlucky. Most likely I was insanely unlucky. If there's something I did wrong please leave a comment below. I may try again in the future when I play Monster Rancher 2 again.