From Quake Wiki
Spawn (also known as Tarbabies or blobs) are annoying little blue blobs that bounce around, ricochet off walls, and hurt you if they smack into you. They don't take long to kill, so don't fire at them unless you're at a safe distance. When they die, they explode like a rocket - and hurt just as badly. They will come after you pretty quickly, so back up as fast as you can and fire once you know you're safe. Don't bother strafing, as they have to be near you to hurt you.
monster_tarbaby is the entity for placing Spawn in a level.
|targetname||The targetname of the Spawn. When triggered, the Spawn will wake up if inactive.|
|target||The targetname of the entity to be triggered when the Spawn dies.|
|killtarget||The targetname of the entity to be removed when the Spawn dies.|
The monster_tarbaby entity supports the following spawnflags:
|1||Ambush||The Spawn will not wake up from seeing other monsters wake up or hearing sounds.|
It also supports the standard spawnflags for difficulty and deathmatch presence.
|256||Not on Easy||The Spawn will not spawn on Easy difficulty.|
|512||Not on Normal||The Spawn will not spawn on Normal difficulty.|
|1024||Not on Hard||The Spawn will not spawn on Hard difficulty.|
|2048||Not in Deathmatch||The Spawn will not spawn in Deathmatch mode.|
This is the format used by TrenchBroom and Worldcraft. Note that the model(...) parameter is not supported by Worldcraft. This does not include the Monster base class definition, which is also required.
@PointClass base(Monster) size(-16 -16 -24, 16 16 40) model(":progs/enforcer.mdl") = monster_tarbaby : "Spawn" 
This is the definition format used for most old Quake editors, including the original QuakeEd.
/*QUAKED monster_tarbaby (1 0 0) (-16 -16 -24) (16 16 40) Ambush */
- The spawn was previously known as the Vomitus, because it was originally going to vomit at you. The bouncing was determined to be scarier, so the name changed. There are still references to the monster_vomitus in the QuakeC.
- "They had been created by Adrian Carmack in response to the imagery of the Formless Spawn in my printed Monster Guide. No one else was using them, so I felt obliged to do so."
- John Romero on Twitter https://twitter.com/romero/status/597079711341158400
- Sandy Petersen on Reddit https://www.reddit.com/r/IAmA/comments/3t1obw/sandy_petersen_designer_of_cthulhu_wars_doom_call/cx2dub6