A Homosexual. Imagine a man riding a horse. Now replace that horse with a man.

The horse rider is the "Knob-Jockey".

the whale tail is the shape formed when a g-string rides up high over a womans trousers

a person who literally 'jockeys' knobs/penises/cocks/dicks. ie they ride dick all day long. this person can be male or female.

a person, often a derogitory term for a homosexual male, who enjoys riding cock.

A person, usually of the female gender, who has a particular penchant for 'riding' a knob. The term is therefore usually used as a derogatory expletive in reference to men, implying that the said person is homosexual and therefore sub-standard

A homosexual.

Meaning 1.
A Homosexual man that rides the knob/head of the penis to minimize penetration/painfulness during anal intercourse.

Meaning 2.
A anal retentive (tight assed) person that won't do a friend a small favour. The payout factor is that usually during sex the entire penis (shaft and knob) penetrates or is riden. This meaning suggests that via being a knob jockey and not a cock jockey that they are not giving the other party the satisfaction of riding the entire shaft rather than just the knob. Somewhat of a bastard.