While doing some research into anything I could find out about the War Hounds Space Marines, I ran across something interesting.During the Great Crusade, the War Hounds Space Marine Legion was split up into smaller units and sent out into the galaxy.
When the War Hounds were reunited with their Primarch and became the World Eaters, it wasn't the entire Legion - my theory is that the elements that later returned to the bosom of the Empire became the Carcharodon Space Marines.
Well here's my theory in full!
This "Bloody 13th" became the core of the War Hounds and were later reunited with their Primarch who turned them into the World Eaters.
In my theory, out in space the other War Hounds still roamed doing the Emperor's work.
Then, from nowhere "Returning from the black void of space beyond the farthest reaches of the galaxy as strangers to both friend and foe alike, the Carcharodons sided with the Loyalists against those that would would challenge the Imperium of the divine Emperor.The exact origins of this infamous Chapter are hard to pinpoint in Imperial archives as the Carcharodons have left no records of their activities in recent history. They feature nowhere within the open Imperial records, and likewise the Chapter has no officially recorded battle honours." 
A mysterious chapter coming out out space to defend the Emperor. They have no history and no battle honours. As if they no longer looked to their past.
"However, sealed and secured records within the Inquisition may shed some light as to the true origins of this Chapter. The oldest of these ancient treatises on the wars of the Imperium of Man may be found within the pages of the Mythos Angelica Mortis written by the savant-scribes of the Secretariat Militum of Terra late in the 35th Millennium. References are made in this oft vague and incomplete work to a Space Marine Chapter identified as the Carcharodon Astra. They were dispatched into the "...outer darkness...upon an endless voyage...to ravage the foes of Mankind..." and to “…set about the Traitor, the alien, and the Renegade without mercy, and harrow them in their places of strength.” This text stems from a far older work, now lost, dating back to the unknown era that surrounds the Astropath Wars of the late 32nd and the early 33rd Millennia. This is a time now shrouded in myth and allegory, and its true nature has been purged from Imperial history so thoroughly that less is known of it than of the Horus Heresy or the Great Crusade." 
So the history of the Carcharodons goes back to potentially before the Horus Heresy, certainly to the dark times when the Imperium was fractured.
The look of the Carcharodons is also reminiscent of the War Hounds "Those Carcharodons who have been seen often sport arms of ancient providence. They are far more likely to carry weapons such as the ancient Phobos R/017 Pattern Bolter or the Mark V Heresy Pattern Power Armour from ten thousand years in the Imperium’s past, alongside other weapons and gear that span the entirety of that ten millennium history."
This might seem a bit tenuous, but have a look at how the Carcharodons use bronze on their armour - as a sign of rank or honour. The same way the War Hounds and the World Eaters do.
Similarly, the War Hounds are described as "developing a reputation for victory, although at a cost, and it was said every assault they conducted ended in only one of two ways: victorious slaughter or simple slaughter, either of which left the foe in no condition to resist further." 
Again - very similar to the War Hounds and the very reason the War Hounds were sent into deep space so long ago!
The Carcharodons also favour the chainsword and chainaxe, so often associated with the War Hounds (and later the World Eaters).
I could be completely wrong - but the idea of the remnants of the War Hounds coming back into Imperial space and being told of their Legion's betrayal then siding with the Emperor, appeals greatly.
Also, if the Carcharodons are meant to be a 23rd Founding Legion, this was the time the Emperor needed as many loyal marines as possible - why not get some of his fiercest warriors from the past?