Q: If a Creeper says I can get rid of it if I “discard a Keeper” (specific or non-specific), does this mean voluntary discard, or involuntary discard?

…does this mean the player can choose to discard a Keeper with the intention of getting rid the Creeper OR the Creeper may only be disposed of if other cards caused the player to discard a/the Keeper.

A: It’s voluntary. You have to choose to sacrifice your Keeper in order to gain the benefit of getting rid of a Creeper. You don’t get the benefit if it’s some other random thing that makes you lose a Keeper.

On the plus side, this means you can do it whenever you choose to. On the minus, it’s not some sort of consolation for having to lose a Keeper involuntarily. “I lost a Keeper, but at least I get to get rid of this Creeper.” Nope, doesn’t work that way.

See also: When a Keeper/Creeper (or Rule) says I can do something if I discard or move something, can I do that as a Free Action or…
And also: If I have a Keeper which says I get some benefit from discarding or moving it…