"Penguin" monsters' ATK and DEF are usually unremarkable, instead focusing on their effects. "Penguin" monsters' effects are primarily focused on returning opponent's cards to the hand. Many "Penguin" cards have effects that support returning cards to the hand.
"Penguin Soldier" appears as a character in World Championship 2006 and in Yu-Gi-Oh! Duel Arena, and "Nightmare Penguin" appears in World Championship 2007, in World Championship 2008, and in Duel Arena.
"Penguin Soldier": Can return up to 2 monsters on the field to the hand, including itself if it wasn't destroyed by battle when flipped (such as when Flip Summoning itself). Especially effective against Synchro, Fusion and Xyz Monsters, since they go back to the Extra Deck, which means the opponent loses field advantage without gaining hand advantage. Since the cards are returned to the hand, this is useful to remove monsters when effects make destroying them difficult, such as "Legendary Six Samurai - Shi En" and "Stardust Dragon".
"Nightmare Penguin": Much like "Penguin Soldier", this card can bounce many of your opponent's stronger monsters. This card can only target 1 card, but it has a basic 1800 DEF, and might withstand an attack which "Penguin Soldier" may not. It is also Level 4, which makes it a perfect target for "The Great Emperor Penguin".
"The Great Emperor Penguin": Can be Tributed to Special Summon up to 2 other "Penguin" monsters from your Deck. With its effect, many power-plays are enabled, and therefore it is currently considered the "Penguin" Deck's strongest card. Its effect can be combined with "Lemuria, the Forgotten City" to raise the levels of the Summoned monsters to Xyz Summon Xyz Monsters with higher ranks.
"Puny Penguin": "Penguins". It's mainly useful in bringing back the likes of "Penguin Soldier", "Nightmare Penguin", or even the "Great Emperor" himself to reuse their effects.
"Nopenguin": Useful for Rank 3 Xyz Summons, because it Special Summons itself from the Graveyard whenever the effect of a "Penguin" monster is activated, but only once per turn. While it can only be Summoned with its effect once per turn, Special Summoning it each turn is still useful each turn, as long as you have "Penguin" effects to activate. In addition, it banishes monsters that are returned to the hand by "Penguin Soldier" and "Nightmare Penguin".
"Penguins" focus on bouncing cards back to its owner's hand with cards like "Penguin Soldier" and "Nightmare Penguin". Cards like "Neo-Spacian Grand Mole" can be used in this Deck to keep with the bouncing theme. You can also use "Nopenguin" or "Degenerate Circuit" to banish bounced cards instead of returning them to the hand. "Spiritual Water Art - Aoi" can dispose of monsters that you returned to the opponent's hand, so using it after using "Penguin Soldier" or "Nightmare Penguin" is a great combo to get rid of strong monsters. By using "Book of Moon", you can re-use these effects to return cards to the opponent's hand again.
"Rescue Rabbit" can be included to bring out either "Bolt Penguin" or "Flying Penguin" to Xyz Summon either Rank 3 or 4 Monster, which can also be brought out by "Great Emperor".