Any equivalent load circuit when an animal is connected to a current injector?
There are models, and standards that define them and tell you what usage the model is valid.

This report from Underwriter's Laboratories (UL) show several models and refers to the standards defining them.

Example:



That image shows a couple of models, with the defining standard given in the text.

Basically a bunch of resistors and capacitors to simulate the body.

There is a different, simpler model that is used for electrostatic discharge tests.  It is used to simulate what static discharge from a human body can do to an electronic circuit.

It is just a capacitor with a specific capacitance charged to a specific (high) voltage and a resistor through which the capacitor is discharged into the device under test.