We know the void lords created the old gods. Perhaps the titans, in an effort to understand how they were made, created one of their own to better understand their true enemy.
Although it is important to note that Chronicles never actually says how many landed on Azeroth, just when Aggramar found the planet, he only saw 4 Old Gods. C'thun, N'zoth, Yogg-saron and Y'shaarj
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We know there are more old gods out there. We have known since TBC where the Sethekk cult tried to summon one to Draenor but Gul'dan killed them and we put an end to it for good.