I use Ulysses to organize most of my D&D stuff; here’s the basic Markdown template I use for monsters (some additional items can be added for more advanced monsters, such as those with special abilities and spells).
*SIZE type, alignment*
**Str** | **Dex** | **Con** | **Int** | **Wis** | **Cha**
**Senses** passive Perception X
**Challenge** X (XX XP)
***Melee*** *Melee Weapon Attack*: +X to hit, reach X ft., X creature. *Hit*: X (XdX+X), X-type damage. (Effects)
***Range***: *Ranged Weapon Attack*: +X to hit, reach X ft., X creature. *Hit*: X (XdX+X), X-type damage.