An emu is a Herschel loop which does not emit gliders. Emus have been constructed for period 52 and for periods 54 and higher (the first few are oscillator-assisted). Quetzals have also been constructed for each of these periods.
See Simkin's p60 for a small example at the size of only 33x31.
The name derives from the fact that they are flightless (incapable of emitting gliders). This term was coined by David Buckingham.
Emus as reflectors
Emus can be used to reflect gliders in a variety of ways. Adam P. Goucher (with help from Jason Summers) exploited this property to construct the first period-59 gun. Unfortunately, that method does not generalise to the remaining emu-only periods, since it uses a special p59+15n reaction based on David Buckingham's p44 oscillator.
- Adam P. Goucher (March 3, 2018). "Re: Oscillator Discussion Thread". Retrieved on March 3, 2018.