Okay, Tripod Synths, as I said, are aliens that are part machine, and are used for military purposes. However, not only military sources use them, but others can also buy them. To buy them legally, they have to be bought off an official port which breeds/makes them, which is owned by the Intergalactic Council, and to buy them, you must fill in a bunch of paperwork with it, things like you are responsible for everything it does, and how you swear not to use it to commit crimes and things about insurance and ownership and whatever. However, they can be bought off the black market, and are popular among pirate ships which are able to afford them. Though they are military devices, they are also legally treated as pets, for they are actual living creatures.
They are well renown for their swiftness and endurance and their weaponry, making them more useful than most soldiers or even many combat drones. Plus, they come in variety. This wide variety is meant for being able to use each type for different purposes. Each type has their own strengths. They are very intelligent creatures, but are still considered "creatures".
Hunters - The most common and most popular Synth. Hunters, also known as Scouts, are known for being extremely swift. Their weaponry can include Pulse Cannons (a type of compressed machine gun fire), Shotgun Shots, Explosive Fléchettes, Plasma Fléchettes, These weapons are shot out two vertical blue barrels, which many people mistake for their eyes. They can become as fast as gazelles, and are just as agile. Some have been known to be able to jump a story or two high to get where they need to, but, they aren't that strong when it comes to things involving the air. They fight best in areas with a lot of obstacles, whether it be indoors or outdoors. They'll use their Explosive Fléchettes if they need to open up areas, for they aren't good at squeezing through tight spaces. They can continue to run and fight for long periods of time, and can easily resist explosive damage. Their weakness however, is energy based weaponry and extreme heat. They are still affected by bullets, if you shoot where they are vulnerable. They sound something like dogs, but in a highly electronic voice, and often 'bark', screech, yelp, whatever.
Pyros - These are much like Hunters. They aren't as fast as them, they are pretty fast anyway. Pyros have a lot of either fire weaponry or plasma weaponry, or sometimes other kinds as well. They are good for support or for special missions. Their armor is resistant to heat and electricity, but however, is weak to explosives. They sound quite similar to Hunters, but also do a lot of moaning.
Bombers - These are quite different than most, being in heavier artillery. They are twice as large as normal Tripod Synths and are shaped quite differently. They use a lot of explosives, mostly chemical based. It shoots cylinder shaped grenades, sticky bombs, rockets, whatever. The arms on their undersides are meant for moving debris out of their way or out of the way of others. They sound like Pyros, but sound deeper, and don't do a lot of barking, and do more snorting.
Treants - Pretty much the most powerful of the pack. Treants are very different from most Synths. They may have Pulse Cannon fire, Minigun fire, Warp Cannon Fire (Highly explosive charged shots), or whatever you can fit on it. With their size and appearance, one would underestimate it's capabilities, thinking it's just all firepower and nothing else. Though they may seem slow at walking, some have been known to run, and make a lot of noise when they do. One may think that it can only attack at a certain height, but with it's giraffe-like stick legs and flexible joins, it can attack at any height, simple by crouching. They are extremely resistant to all types of danger, and have very strong balance. They sound like whales.
Stalkers - The best type for assassinations. They, unlike most of their louder kin, are able to keep themselves quiet, perfect for scouting or spying missions, but will occasionally hiss, moan, yelp or snort. They use their blades more to their advantage, and their blades are better hidden than those of Hunters. They are also experts at targeting enemies and hitting them, much like snipers, but with silences gunshots. They will sometimes use Pulse Cannon fire for self-defense, if they ever need it.
Turrets - Turrets have the most technological upgrading. They aren't as mobile as most Tripod Synths. They prefer planting themselves on the ground and relentlessly firing anything that moves. They can have Pulse Cannon Fire, Minigun fire, Rockets, Sniper shots, Plasma Shots, and can deploy Hover Sentries. Hover Sentires are tiny mechanical devices that fly using a small fast rotating blade that can be used to seriously harm an enemy, and are able to sense heat, so they are usually deployed by soldiers wherever they can't get to. Turrets are very sturdy, and once planted, they will almost never be pushed back. They make strong for defensive based combat, for at the same time they are good at defending against ground attacks, they are also good at defending against air attacks, unlike most Synths (other than the Treant) though some of it's weapons can be slow when used against said targets. They are very bad at defending themselves from behind though. They sound like Hunters, but a little bit more high pitched and even more electronic sounding, and also make a lot of beeping sounds.
Healers - Healers are about as agile as Scouts, which is essential for their job. Their job is to heal and recharge fellow Synths. They cannot heal anything else other than Tripod Synths, however, they are still able to charge things. They have a special piece of equipment that works similar to that of a tracking beam, which can be used to speed up tissue healing of Synths. It can also be used to recharge a Synth's mechanical body. The only have Pulse Cannons to defend themselves with, though, their armor is exactly the same as that of a Hunter. A Healer doesn't have claws like a Hunter does, but has arms along it's underside in the same way that a Bomber does.
It is unknown to the public the exact history of these creatures, nor how exactly they are bred/made nor where they originated from, however, they have been accepted as loyal military "pets".