Zeta Metroids are Metroids which have grown past their Alpha Metroid and Gamma Metroid stages. They are followed by Omega Metroids and, if the Metroid has the proper genetic coding, Queen Metroids in the Metroid life-cycle. Zeta Metroids attack by firing projectiles and by ramming.
In Metroid Fusion aboard the B.S.L research station, Samus finds a single Zeta Metroid in a stasis tank in the Restricted Laboratory of the Restricted Zone in the Main Deck. This creature is found alongside other Metroids from different stages of the life cycle. This Zeta Metroid is never battled by Samus, and it is probably destroyed with the entirety of the research station.
A Gamma Metroid and a Zeta Metroid in stasis
A Zeta Metroid molting from its Gamma Metroid form
Artwork of a Zeta Metroid for Metroid: Samus Returns