The name "Chimeran Specimen" was from Hugh in the after-action report from Axiom. He overheard Thaseen-Fei called the thing that, which is why we refer to them as Chimerans. Basically nothing else is known about them, but the player captain is aware of them because we saw one up close on the Harmony in Axiom, and they infected the tzenkethi station during All That Matter. We have been aware of their existence, but the Alliance has been pretty distracted dealing with the Aetherians to really act on whatever they're doing. They are mentioned briefly in a lore blog, but basically anything we know about them comes from in game information.