Phoenix Manticore BIOS, sometimes also called just Phoenix Manticore, is a version of PhoenixBIOS developed by Phoenix Technologies. It is briefly mentioned in a blog post made by Tim Lewis, an UEFI firmware engineer, on uefi.blogspot.com in 2014. The blog post itself is a redirect to another blog post made by Vincent Zimmer, reviewing changes in the UEFI specification since its initial release by Intel in 2004.
It is also mentioned in a patent application made by Phoenix in 2001, which is the continuation of another application that was made in 1998. This patent contains file listings from the source code of a BIOS for the Intel 440BX chipset (possibly Manticore) that are dated around 1999, meaning Manticore was likely either released or in development in around 1998-1999. Aside from this, not much is known about Manticore and it is unclear whether or not it was ever released. Based on its name, the dates in the file listings mentioned above and the scarcity of information, it may have been the codename for a planned successor to PhoenixBIOS 4.0 Release 6.0 that was later cancelled. However, there is no evidence to confirm this speculation.