Robot tracking a moving object (that can carry some sort of transponder)

I'm considering a project that would consist of a autonomous robot following another moving object (like a dog following a person). The object being followed can carry a device/transponder to simplify this project. I know this could be done optically, or by wirelessly communicating position from the followed object to the following robot, but I figured that there must be a simpler way to do it, but I can't think of a sensor suite off hand that does this.

Requirements:

Can lock and initiate following at a distance of 50m or less. Low power. Fast (e.g., could account for the followee zigzagging while running)

Does any one know of any ways to do this?

[first post: be gentle :)]

Check out "Hop, the Following Suitcase" https://www.youtube.com/watch?v=hucNBe5sgY0