The sixteen bytes SPAWN_MONSTER_INTO_ROOM copies into a free slot, every one of them meaning something documented elsewhere.

The sprite is $58 -- not a creature but the arrival animation, so a new monster appears by materialising rather than blinking into existence. +$03 and +$04 are the centre of the room, overwritten by RANDOM_POSITION. +$08 and +$09 are its starting velocities. And +$02 is the creature to become when the animation ends -- the same trick the player uses when dying -- though the template's own value there, the spider, is never used: the spawner always writes a code from SPAWN_TYPES over it.