Did "slave" ever apply to anything other than people?

Solution 1:

The collocation (and meaning) slave device goes back well before electronic devices. The OED gives examples of meaning C1 c. ("Used to denote a subsidiary device, esp. one which is controlled by, or which follows accurately the movements of, another device.") from 1904.

Solution 2:

Ants take slaves:

Slave-making ants are brood parasites that capture brood of other ant species to increase the worker force of their colony. After emerging in the slave-maker nest, slave workers work as if they were in their own colony, while parasite workers only concentrate on replenishing the labor force from neighboring host nests, a process called slave raiding.

You can argue that cuckoos enslave other birds to raise their young as well.

Solution 3:

OED lists “slave jib” or just “slave” as a sailing term from 1934, meaning a jib that was pretty much permanently set.