One of the most profitable ways to build up money on Runescape 2 is to hunt. There are two great "monsters", if that is what the little things are.
They are Red Chinchompas and Dragon Implings. Each one could make you anywhere from 200k to 3000k per day.
Red Chincompa ---> <----- Dragon Impling
For Red Chinchompas, you need to have level 63 hunting. The place to hunt them is straight south of Yanille, the city which is east of the Castle Wars mini-game. Walk south from Yanille's east entrance and you will go through a narrow path and into a forest area. Keep walking south and you will come to Red Chinchompas.
Make sure to bring about 3-5 traps, depending on your level, and you may want to bring some more, in-case some end up disapearing. Don't worry about having space for Red Chins. They stack up on each other the same way as arrows do, so all you need is one space in your inventory open to build up money! Usually, you can catch anywhere from 100-200 chins per hour, but this all depends on your level, the number of traps you have, and how well you pay attention. Each Red Chinchompa caught will earn you around 740 gb each.
The second hunted monster you want to go after for this Rs guide is Dragon Implings. You need 83 hunting or 80 if you decide to use hunting potions. These will get you around 300k each! You will need to head over and play the Impetuos Impulse mini-game, which is located in Zanaris. Zanaris is a town located near Lumbridge, but it can only be accessed after you have completed some quests and obtain a Dramen staff. You can use a regular net or a magic net to catch them, and you will need to put them into impling jars. They respawn about 1 every hour, which makes them sort of difficult to catch them fast. You will need to have a good amount of patience to do this often; however, you will definitely become fast!