Anyone who has ever been a new player in Ruenscape is familiar with cows. They are the top mobs to train on because of three very useful drops: bones, raw meat and cow hides.
The bones, of course, only give a little bit of prayer exp but when you are low level, a little bit is all you need to level. The raw beef is good for training cooking and also it heals some hitpoints so you can train at the cow area practically forever just using the drops to heal. Lastly, the hides can be gathered and banked to use or sell later. These cow hides are a very easy way to make money for new players, especially in free to play.
The only problem with this is that the f2p worlds tend to be really full of people who are trying to kill cows. The good news is that in members there are more cows and fewer players so that is one good reason to get members. In fact, when you first log into the tutorial area in p2p one of the first things you will do is kill cows, get cowhides and craft them into leather gloves. The bank is also nearby for those who want to sell the drops to other players in the Grand Exchange.
As for me, I have been killing cows for days now because I need to level up crafting so that I can do the quest to get the dragon long sword. The dragon scimitar is better for training in p2p but that quest is probably too hard for me to do for now. My current goal is to train until level 60 attack, do the quest, get the dragon long and then work towards getting s scim.