Imagine if you could have a cute little pet, following you around the field of battle, lending you some much needed help. Like a Mecha Knight, but better.
Basically, anytime you fight a monster, they now have a slight chance of dropping a certain token. These tokens represent each monster type (fiend, slime, etc.) and can be traded at a special token trader.
To summon a monster you own into battle, you must have a 'pod' (like a Vitapod). The pod works like a Vitapod, it will be stuck to you for as long as you don't return to Haven. As long as you have a pod on you, your pet will help you in battle.
There is a number on the Monsterpod. The number decreases everytime your monster gets killed in battle, but as long as the number isn't 0, your monster will always revive automatically.
When you trade your tokens for a monster the first time, you will receive a basic t1 monster. You can obtain any monster except the compound monsters (glop drops, dust bunnies, scarabs), battlepods, phantoms, healers, curse-monsters and Trojan-like monsters.
Having a gun puppy, howlitzer or polyp as a pet will put them on a floating platform. Allowing them to move and follow you around.
At the end of every battle, your monster will receive heat. Unlike sprites, you can't feed your pets. They level up automatically every time their heat maxes out.
Also like sprites, they can be evolved at certain levels (15, 49 and 100). The evo device needs to be bought using energy. Level 100 is the max level.
Your monster will follow you around Haven and any other place where your weapons can't be used. (They'll disappear in the lab and the hall of heroes). You can also talk to your monster in the Ready Room.
In the case where a person owns multiple monsters, they can choose which one they take into battle by acessing the arsenal and equipping them. Like sprites.
....And that's everything. I think. I may edit in things and fixes laterin the future.
Comments? Suggestions? Insults? Don't be afraid to speak out.
Back in the day before we had the battle sprites, every three days someone would come on and post something along the lines of-
"There could be cheap pets which are selled in the ac or the merchant. OR there can be rare/more powerful pets dropped from bosses from like missions." -Japstar
Then it got really old and became a sort of taboo, much like posting about the gunner update in general discussions is today. Aureate points out exactly what I'm saying here- http://forums.spiralknights.com/en/node/47077#comment-309631
Despite how unpopular the pet suggestions were, one made it into the game, and became battle sprites, and this is the link that you really do need to click on.
http://forums.spiralknights.com/en/node/68828
The reason this one made it into the game, was because it dealt with all of the issues that people on the forums had discussed previously, and seemed to solve them as best as possible. So in a very large way, your idea is already implemented as best as it can possibly be, in the form of battle sprites.