Needs a rider?

Possibly. "As long as they control more than one" would be thematic.

I don't know, the whole "run faster than you" thing tells me that if you're all by yourself in a situation where the hindmost gets eaten, that probably doesn't equate to "you're in no trouble".

I think I'm not sure, this effect by itself isn't that interesting, it might benefit from a small advantage or downside; an extra hoop makes it even narrower.

Doesn't need a rider due to power level concerns, but thematically the above suggestion seems nice.

If you want to restrict itto make it more interactive, you could go with "sacrifices a creature with the greatest toughness among creatures they control." You know, because tough guys are slow?

I realised this after commenting on Exviscerator, but since the flavor comes from marching order, there are options like "Target player sacrifices a creature that has[n't] attacked this turn." (instant) or "Target player sacrifices an [un]tapped creature." or "Target player sacrifices the last creature that entered the battlefield this turn." (instant) etc.

I really like the simplicity of sacrificing a creature of their choice representing the hindmost. I'd like to keep that. But maybe make the card slightly more interesting by adding some other effect if I can find one that fits the flavour, like gaining or losing a little life, etc.