Runescape is a very large game with tons of different monsters. I have compiled a list of the most profitable ones to kill and a rating ( out of 10) on how profitable it is to kill them.
Green dragons- Lv 79
Green dragons are a very profitable to kill for lower levels. They are weak and will die fast. Be sure to bring a dragon fire shield and either a cross bow or your melee weapon ( mage is ineffective i feel) and if you have the money an anti dragon fire potion. Don't take too much food with you as you'll need to collect the bones to sell and hide to sell. The drops to look for are- hide, bones, level 3 clues. The only downside is there found in the wilderness.
Steel/iron dragons- LV 246/189
These are personally my favorite monsters to kill for money. Bring a dragon fire shield and 4-5 anti dragonfire potions. If your ranging i suggest 75 range and addy bolts (e). Magers need 75 as well and use fire bolt along with klanks gauntlets ( from the family crest quest) drops- bones ( bury from here as you'll be here for a while) dragon legs and skirt, dragon visge, lots of rune drops.
aviansies- lv 69-148
These are another great money maker but are in the godwars dungeon so learn a little about the godwars dungeon before you head there. Bring a rune c bow with any type of bolts and some prayer pots to stay there for quite a long time. These drop quite a lot of adamite bars which sell for 4k each ( in an hour i can get around 100). Also bring some runes for high alcing. They drop a lot of rune daggers to alc for atleast another 100-200k per hour.
Mithril Dragons- lv 304
These very strong dragons are found in the ancient cavern which requires a small portion of the barbarian quest to be done. They are very strong and require 75+ range ( 80 if you want to kill more than 1-4 per trip). Melee and mage don't work on them. The are very hard to kill. You must use a dragonfire pot, protection from range, and stand well out of melee range from em. Be on the look out for the dragon full helm and charred bones ( which can be burned for a dragon full helm.
These are four of my favorite monsters in runescape. They drop a lot of gold, items, are well designed creatures and just fun creatures to go out and kill. I hope this short guide has given you a little more insight on the game of Runescape. I also hope it gave you some good tips on monsters to kill to help get you a little extra gold.