From Quake Wiki
Fiends (also called demons in the QC code) appear almost small the way they position their bodies at rest, and not much larger when walking, but when you see them run, let alone leap, you will appreciate their size - and ferocity.
Fiends make their first appearance in Castle of the Damned as a literal monster closet, leaping out at you while you observe the floating stone blocks. They are quite potent in their slashing, but also can take a handful of rockets. However those hooved legs should be your concern. The leap is fast and powerful (bare minimum of 40 damage) and quick to recover. Be swift on your feet, particularly with sidestepping and take advantage of its exposed back during the jump. If you can keep close enough in range, they will generally not leap, but they do take small steps forward as they slash, so be careful.
monster_demon1 is the entity for placing Fiends in a level.
|targetname||The targetname of the Fiend. When triggered, the Fiend will wake up if inactive.|
|target||The targetname of the entity to be triggered when the Fiend dies.|
|killtarget||The targetname of the entity to be removed when the Fiend dies.|
The monster_demon1 entity supports the following spawnflags:
|1||Ambush||The Fiend 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 Fiend will not spawn on Easy difficulty.|
|512||Not on Normal||The Fiend will not spawn on Normal difficulty.|
|1024||Not on Hard||The Fiend will not spawn on Hard difficulty.|
|2048||Not in Deathmatch||The Fiend 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(-32 -32 -24, 32 32 64) model(":progs/demon.mdl") = monster_demon1 : "Fiend" 
This is the definition format used for most old Quake editors, including the original QuakeEd.
/*QUAKED monster_demon1 (1 0 0) (-32 -32 -24) (32 32 64) Ambush */