Maybe for an UU.
It is a catapult with a very weaker attack, compared to normal onagres. But, the projectile should have an additional effect. Here my suggestions:
It could damage the garrisoned units inside Rams, towers, ships and buildings. A normal onagres would destroy a town center in a much faster time. But, a corpse catapult would harm the villagers inside the town center with every shot, killing them after a number of shots. That forces your rival to evacuate them before the TC destruction.
It could create an area of effect with damage on time. But this could be complicated, and it would change a lot of the gameplay.
It could poison the units attacked, with a small damage on time for the next minute. The units could garrison to try to survive. But normal onagres would just kill the units, which is better.
It just has a bonus against Calvary and ships. But the normal onagres are that strong that it don’t need any bonus against those units.
I consider the first option the one that gives the unit an original niche, that can be useful in raids.
The other options would be cool if the corpse catapult had a cheap prize, not like the onagre. It could even be considered a trash siege.
There were some civs that used biological warfare in this sense. Turks and tatar are good examples.