I was just sitting around thinking about what models in Arena Rex currently represent the only example of of a "type" of models, like something outside of their own personal special rules.
For example, I thought that Wendigo was the only model in the game that is not connected to any specific ludus benefits card. But that's not true - neither is Noxius.
I have one though: Stheno is the only 50mm based gladiator in the game so far.
Nero is the only mountable beast that has two stages. And Caho-Ta is the only mountable beast that is not a titan. (Which shows that the other two mountable beasts are both one-stage titans.)
I guess two of the ludi only have one 40mm gladiator - Morituri only have Ur-Kek, and Legio only have Gaius. The other two ludi have two each.
I'll see what else I can come up with...