Wrangler usually refers to Maths.
In my experience, anyone with a first in Part II (third year) Mathematics is called a "Wrangler" and then the order in which people finished extends the terminology to "Senior Wrangler", "Second Wrangler", "Seventh Wrangler" etc. Someone I met was 33rd Wrangler.
The Cambridge system for maths is inherently competitive. As far as I have understood it, I will not find out my percentage at the end of this (my first) year. I will only find out where I was placed within a class. For instance people last year could assume they were a low first because they were 50th on the list (or something), but theoretically they could have had a very high first but there were forty-nine others who did incredibly well that year.
I believe it is either unique, or within a definite minority of subjects whose results are comparative and not objective.
note: They read out the names of the Wranglers in public from Senate House when the finals results are announced, in order. I'm not sure that this happens for other subjects.