at the moment, let's just call that lego muscle-mouth-cow Species A.
If we figured out hugely advanced species, we'd have to figure out culture, customs, architecture, aesthetics, etc.
Here's an idea for a species I thought up, that requires no extra art whatsoever (yay!). Species B starts it's lifecycle as a single-celled organism. (note: we don't have to put all this life-cycle stuff into gameplay)
Unlike most other single-celled organisms, it has some intelligence. It adapts to it's environment in ways that predict and adapt to future changes. These organisms enter into an animal body (human, or otherwise) through various ways. Due to their intelligence and general-purpose abilities, they can adapt to a wide variaty of hosts. The easiest is into wound of some sort. Sometimes they are capable of sneaking in straight through the skin (this is rare, however). When eaten, they usually do not survive the stomach acid.
Once they enter the body, they disguise themselves as the cells of the host organism by copying various protein markers and other chemicals, rendering the cells almost invisible to the host's unsuspecting immune system. They feed on a wide variety of cells, and begin growing into a multicellular organism. If several seed cells are present, they will either attempt to fight it out, or merge into a single organism, this varies from case to case.
The organism replaces existing cells in the body, starting with whatever is closest. They do not feed upon major organs, because those are needed for the survival of the parasite as much as the host. At this point, the parasite will destroy any other invading infection and help healing of any physical condition (if it can, some things it cannot do), to allow for a more suitable host. As the organism eats, it goes in a specific direction: towards the nervous system. It replaces nerve cells with itself. The victim usually feels at most only a slight tingling sensation and some clumsiness, as for the time being, the cells act as if they are part of the host's body. They follow the nerves up the spinal cord, and into the brain. Once the parasite reaches the brain, it disables human control of all of the nerves in the body, to prevent resistance (at least the voluntary functions of the brain, heartrate and stuff like that are preserved). The parasite then consumes the brain and replaces it with itself.
The parasite then essentially is a replacement brain. The parasite and the host are now essentially the same organism. The organism will have no knowledge of the old host, it just has to attempt to stand up and relearn to walk and do other basic things that we learned as infants. Once the organism can walk, move about, and eat, it begans producing single celled seed cells. These are usually found in the organism's saliva or other bodily fluids, it varies from case to case. The organism eats the same things that the original host organism eats. The organism will then attempt to infect other potential hosts.
If a syringe is availible, that is preferred. At other times, the method is usually just to lick a knife or other sharp object and spit on it, and then stab a potential victim. They use disturbingly clever strategies and stealth to infect victims. When in groups, they employ group strategies, through some kind of egokinetic communication. They are unable to speak (this is not clear whether this is due to a lack of needed intelligence, or simply because they do not consider it useful). They have been observed making odd noises with their vocal chords at times, but usually this serves a purpose like distracting a target, or some other similiar thing. They are capable of using tools, even advanced tools like guns. The organisms usually confuse potential hosts with weak egokinetic abilities.
The only way to reverse the process is by the help of a powerful biokinetic, and even they have difficulty regrowing nerve cells. An egokinetic can slow or damage the parasite, but he cannot replace damaged cells, or destroy the organism completely.
note: I'm not really propose that we implement the ability for the player to be infected with one of these, perhaps some NPCs would have it.