Source: https://www.wisdomlib.org/definition/rakshasendra. It says it means Rāvaṇa (I find this unlikely), or Kuvera (which would bring about more questions, but will also answer a few). Where in the Mahābhārata is Rākṣasendra mentioned?
From searching online, I find the majority of the "sources," claiming Rāvaṇa is Rākṣasendra claim [Ann.111] as a source. These sources also claim that Rākṣasas are aboriginals, which does not make sense for a 101 different reasons, ranging from they are way, way too big (e.g. in the Kalki Purāṇa) to their ancestry clearly being different to humans. Thus, I'm doubtful this source is a good one.
e.g Brahman: The Discovery of the God of Abraham By Prof.M.M. Ninan
The book A Classical Dictionary of Hindu Mythology and Religion, Geography, History ... By John Dowson mentions Rākṣasendra as Kuvera (twice). Does anyone have the full quotes from the book? (I only saw snipets)
Also, does anyone know if the idea that Kubera and Kuvera are the same is surported in scripture, or are their descriptions just similar due to Rākṣasas and Yakṣas doing everything together?