If we have Giant Frogs doing 5 damage, I'd say it's safe to give an Emperor Penguin 4 damage. Especially considering that for a frog to be giant it could still be half the size of an emperor penguin.
Yes, but this is a fantasy game and we have no way of knowing the actual size of a "Giant Frog". For all we know it could be as big as a house. Also we don't know how it does damage. Maybe it has some kind of an attack what does not require a big size.
This is different with Penguins because Penguins are not fantasy creatures. We know exactly how small they are, how little they do damage, and how non-aggressive they are. Not only do these Penguins here have a relatively high starting attack, their ability makes them even more aggressive and strong. You made them offensive creatures, when in real life Penguins are anything but that.
Thematically, the best way to use Penguins would be to make them support creatures with a low attack. Making them offensive creatures like this just doesn't make sense to me.
Then perhaps you could explain the Antlion card? As you probably know, an Antlion is a real-life insect, and yet it deals as much damage as this. I'd say a Penguin can do as much if not more damage with an Antlion.
As you said, this is a fantasy game; just because a creature exists in real life does not mean we are restricted to the real-life characteristics of the creature. The Antlion and the Penguin are both magical.
Well, not that many people know what Antlions are, Antlions in Elements look nothing like the real Antlions, and Antlions are defensive creatures, not offensive.
Penguin | Emperor Penguin combined with that image is a very strong evidence that we are talking about just your average Penguin. If you see them as "Magical Penguins", you are more imaginative than I am. I see them as regular Penguins because nothing suggests otherwise and I don't see regular Penguins as offensive creatures at all.
Instead of trying to desperately find arguments why the original theme is perfect, why not just change it to something that fits better even without the "Magical" part, like that Polar Bear Hyro suggested? I mean it's not like mechanically this idea has anything to do with Penguins. Other option is to just ignore feedback and go with what you think is best. Nothing wrong with that.
Well, it will stay a Penguin unless I get art for something else such as a Polar Bear. Plus, if this card were to make it in the game and changed slightly to be more thematic, it wouldn't be the first time.
Polar Bear sounds much better, although someone could argue that then it would need more attack, seeing how big and strong Polar Bears are. If you want to make it less obvious, you could go with something like Arctic Fox or Arctic Wolf.
I messed around with Photoshop a bit, but it was pretty difficult to work with those white coats.
I'm not really happy about the final results because they look too realistic, but if you (or anyone else) wants to use these for something, be my guest. Definitely not suitable for Elements, but could be used as placeholders.
I would like to add that I do not really care what the theme is and I won't lose sleep over Penguins.
But I will say that I like that cute Penguin art and I would love to see it as a support creature.
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Sanctuary card and converts it to a 100% 
to 
was a good move. Makes more sense this way. Not sure about the mechanics though. Maybe you should try what Zblader suggested. Like... generate a copy when the owner plays a