I'd say it would be a closely guarded secret that Tarantulas and Tripredacus are not Children of Primus.
<<<SPOILERS AHOY>>>
References are:
1/ When Tarantulas was attempting to destroy the Ark, it was mentioned that doing so would kill all the Autobots and Decepticons inside, thus destroying both Maximals and Decepticons. Tarantulas then said that it would only effect Transformers descendant from Autobots and Decepticons and that he and the Tripredacus Council have different origins.
2/ Megatron once referred to Tarantulas as "Unicron's Spawn."
3/ We know that during the Beast Wars era Unicron was engaged in a quest to rebuild himself by gathering Angolmois Energy (aka Dark Essence). The Blentrons were his heralds and he also possessed Galvatron, Shokoract and even Vector Sigma. He eventually tried to convert Cybertron into his new body, similar to what he did in G1 during "The Legacy of Unicron" when he enslaved the Junkions to rebuild Junk into his new body.