An interesting theory that has been tossed about. But, the T5 monster being a Basilisk Soldier would ruin the entire mysterious lore behind Evolve’s monsters. It will evolve in a chemically cocoon like the other monsters, eat and gain armor like them, come from an egg like them, etc. That would imply that all of them are some failed Basilisk Soldier experiment.
That also means if they were Basilisk, I’m pretty sure someone would have noticed. We have some Hunters that fought or knew Gen 1 or 2 Basilisk Soldiers (including Lazarus, Torvald, and Hyde), so I’m sure they knew how they worked or what they looked like. We also have the nerdy scientist Caira, who has been studying the monsters and what their bodies are made up of.