I consider that in Fantasy RPGs, those that use magic are not normal anymore. Like those that worked with radioactive materials in the past of the real world, the powers they use can twist and deform them, mentally and physically. Add to that the intense competition between peers and rivals, and you must admit that their perceptions will most probably change.
For some, any benefit, or any special or unexpected benefit, might suffice. Others will bond with purpose, or perhaps with the one animal that does not fear, or perhaps, scare, them.
More on this later. For now, a brief explanation of what I'm trying to achieve here, and some more beasties;
Whatever system you use will probably have a system for familiars. If not, improvise. The animal gains hit points with their master, perceptions, health, and magic can be shared, and special abilities either become apparent, or perhaps controllable, at certain levels.
The Rust Monster
These monsters are cute, and purposeful. Although the low level mage is likely to have much in the way of fighter companionship, there are advantages. Good for a druid?
- At low levels, this will not be fun, and you will not be popular. It may eat the weapons & armour of your foes. It will definitely eat your metal objects, and those of your friends.
- At the mid levels, you will have some control over it's metal eating issues. Not control, more like a naughty puppy that will try and get away with anything. But better than before. It will also pause when it first detects metal, kind of like a bird dog.
- At high levels, you can tell it which metals to eat, or not eat, although if it's hungry, and there is nothing else...