Latin for "hammer" or "mallet", a Malleus as depicted in the series is a large, two-handed bludgeoning weapon.
Hammer-style weapons in the series appear in two forms: as large iron 'sledgehammers' and (more commonly) as heavy wooden mallets with a spike on the rear of the 'head' and another on the end of the handle.
In both cases, these are slow and heavy weapons that require two hands to use, but are capable of inflicting bone-crushing impacts and massive damage.
A third style of hammer used not as a weapon but a tool was the common wooden mallet. This short-handled, one-handed hammer was used by Roman soldiers for pitching tents... and for crucifying prisoners!
While not carried by any official gladiator class, weaponized hammers are used several times during the brutal fights in The Pit. In the episode The Thing in the Pit, the savage Ixion fights with a huge malleus ferrum (iron hammer) against Kerza. In the episode A Place In This World, Caratacus nearly slays Oenomaus with a wooden hammer.