Groups that use a single letter for Abbreviations?
 

Members of Alpha Phi Alpha will refer to themselves as Alphas.
Members of Beta Theta Pi will refer to themselves as Betas.

I believe that members of Zeta Tau Alpha will refer to themselves simply as Zetas.

What other GLOs will refer to their members as simply "Single Greek Letter"s as the previous two?

(Note, I don't think anyone refers to themselves as "Gammas" other than the Fictional Gamma Phi Gamma in Spike Lee's School Daze)

Kappa Alpha Theta - Thetas

Delta Sigma Theta- Deltas

Phi Beta Sigma - Sigmas

Kappa Alpha Psi- Kappas

Iota Phi Theta- Iotas

Some chapters of Sigma Sigma Sigma use 'Sigmas'

Sigma Chi uses "Sigs" instead of the whole "Sigmas"

As a quick additional note, I've seen some newspapers from the 1960s with a *few* chapters of Alpha Phi Omega being refered to as Omegas, but I haven't seen it in the last 40 years.

Delta Tau Delta—Delt(s)

Kappa Kappa Gamma—Kappas

Clarion had a local fraternity named Alpha Gamma Phi and they were called the Gammas! They started in the 50s I believe.

Delta Kappa Epsilon - Dekes
Tau Kappa Epsilon - Tekes
Pi Kappa Alpha - Pikes

I know so some Alpha Epsilon Phi chapters refer to themselves as “Phis”.

Zeta Phi Beta also goes by “Zetas” and you better make sure to pronounce that T! :)

Sigma Lambda Gamma refers to themselves as “Gammas!”

Omega Psi Phi often goes by “Omegas” or “Omega Men.”

The women of Alpha Sigma Tau at Western Illinois University have long lived in "The House of The Taus". This differentiates them from members of the WIU Alpha Sigma Alpha house (aka the OTHER Alpha Sigs in the NPC).

(for further reference there are also Tri Sigs, Phi Sigs, DZs and ChiOs at WIU)

I've always thought it odd we weren't the Gammas, as we are the only ones that start with it.

We were Gamma Sig, at least on my campus.

Some chapters of Alpha Phi call themselves 'Phis'

I often wondered that!

But Deke, Teke and Pike aren't letters....

This wasn't the case when I was in school but I have seen it more and more in the last several years, Lambda Chi Alpha being refereed to as just Lambda.