Engineers can create several pets, ranging from non-combat pets to various mechanical damagedealers. These pets are different from regular pets and do not replace your normal pet if you are a hunter or warlock.
Summoned engineering pets are aggressive and will engage any nearby hostile targets, which can make them a liability at times.
Combat Pets
There are several pets that are useful to you in battle. Some of them are usable directly from your inventory, some come in the form of trinkets. Some are infinitely usable, some are one-time use. Note that in general, trinket pets have fallen into disuse, mostly due to the rise of so very many powerful trinkets in BC.
Compact Harvest Reaper (175)
The Compact Harvest Reaper is the first combat pet you can make. It sticks around for 10 minutes, but is not reusable. It can be looted when it breaks down to recover some of the materials used to create it. He is always level 35, and will lose its usefulness as your opponents get higher level.
Dragonlings (200 / 250 / 300)
Dragonlings come in trinket form and are usable once per hour. They will stick around for one minute, after which they die. The level of your dragonlings is determined by your engineering skill divided by 5. They come in three strengths;
Mechanical Dragonling (200)
This is the weakest of the dragonlings. It nifty at when you can't make the higher quality versions yet.
Mithril Mechanical Dragonling
A step up from the Mechanical version, this one does a fair bit more damage.

Arcanite Dragonling
The best Dragonling available, this one does more melee damage and has a fire breath ability. It is was costly to produce before BC, and still is. The schematic is a very rare find from Cobalt Mageweavers in Winterspring. The trinket is BoE, so it can be obtained without having the schematic.
Gnomish Battle Chicken (230)
Gnomish engineers can build the awe inspiring Gnomish Battlechicken. This diabolical contraption usually deals 1 damage per hit, until it casts ’Chicken Fury’ which boosts its attack speed and damage by 100% each. This somehow boosts its damage to up to 80 per hit. It sometimes casts a 5 minute duration buff called Battle Squawk which boosts your attack speed by 5%, which is nothing great but funny nonetheless. It lasts for 90 seconds when summoned and has a half hour cooldown. It levels with you like the dragonlings do, at a rate of 1 level per 5 points of engineering skill.
Non Combat Pets
There are various non combat pets the engineer can make. Some for all to use, some for yourself only. They are all novelty items.
Mechanical Squirrel Box (75)
Summons a small mechanical squirrel. This is learned fairly early, the schematic is a random world drop. If you want to have it, its best to buy it.
Lil'Smoky and Pet Bombling (205)
These two are specialization specific and can only be obtained by sheer luck. When you renew your specialization membership card, you will receive a gift in the mail after 24 hours. There is a small chance that you will have the schematic in that gift. You can only have the schematic that matches your specialization. Lil'Smoky is for Gnomish engineers, the Pet Bombling for Goblin engineers.
Tranquil Mechanical yeti (250)
This one can be learned when you complete the quest 'A Yeti of Your Own' in Winterspring by speaking to the questgiver after turning it in. You must be level 55 and have 250 engineering skill to get it. It binds when used and is not unique, thus can be traded/sold.

Lifelike Mechanical Toad (265)
The rarity of this schematic is on par with pre-BC epic patterns. It is a very, very rare world drop. Your best shot at getting the schematic is buying it in the AH, be prepared to pay up to 100g.
Target Dummies
Target Dummies taunt nearby enemies the moment you use them, and will then proceed to do so in pulses every few seconds afterwards, for as long as they stay in one piece.
Note: they will taunt and aggro neutral (yellow) targets, use with caution.
The dummy will last for 15 seconds or until destroyed. Advanced and Masterwork Dummies have much more hitpoints and better taunts than the regular ones.
Make sure to loot it after it dies, you will get some components back along with a chance for fused wiring.
Repair Bots
Repair bots are used to repair your gear when stuck deep in an instance with no repair capable npc's around. These used to see a lot of use before Burning Crusade was released. In BC it became much easier to reach a repair npc. Most instances have one close to the entrance. And if you're dying and running back, why not one out for a quick repair? Especially considering that repair bots charge up to 20% more gold for repairs than NPC's, because the bots are not affiliated with any reputation..
Even so, the bots are still used occasionally. There are two varieties:
Field Repair Bot 74A
The first repair bot, Field Repair Bot 74A used to see a great deal of the old 40 man instances. The bot offers repair services and sells reagents. The schematic is obtained in Blackrock Depths, it can be found lying on the ground near Golem Lord Argelmach. This is soloa ble by a level 70 by taking the following steps:

Obtain either the Shadowforge Key or several Powerful Seaforium Charges (bring at least 4, to be safe). If you wish to avoid fighting anything, bring a gnomish cloaking device and/or some invisibility potions.
1.Enter the zone and open the door to your left. Go through, open the door to your right. When through that, open the next door to your left.
2.You enter a room with a small ramp leading up to a higher level. Go halfway up it, and if you look down you'll see a large usable gear. Drop down and blast it open.
3.Go all the way up the ramp and head left. Follow the 'road' until you reach a somewhat bigger room. If you keep to your right, you will find a doorway with a ramp leading straight down into an area with non-elite gnomes roaming around. Go straight through this room to reach Argelmach's room.
4.If you stick to the side, you can avoid aggroing Argelmach, but killing him should be no problem. The Schematic lies on the ground in his room.
Field Repair Bot 110G
This is the upgraded repair bot. It is cheaper to make even though it requires more expensive materials, as this isn't a one time use bot. It has five charges. It offers;
●Repairs
●Reagents
●level 65 Shells/Arrows
●A few Super mana and health potions
●Some buff type scrolls
●Crystal and Imbued Vials
The schematic is dropped by Gan'arg Analyzers in the Blade's Edge Mountains.
Alarm-O-Bot
The Alarm-O-Bot is a little bot that will detect stealthed or invis opponents.

With it’s 10 minute duration and single time use, coupled with rather expensive ingredient requirements, I have never seen anyone actually use it. The amount of ingredients required to make one is not proportional to a 10 minute limited benefit.

Minigame Bots
There is only one actual bot that qualifies as a minigame bot; the Tonk. These tonks are the same as the ones you can control in the Darkmoon Faire, but the engineered ones can be used anywhere. Anyone can use these 50 charge items to do some tonk versus tonk combat. When you summon a tonk, your point of view moves to that of the tonk, and a pet bar with the tonk abilities appears. This makes tonks somewhat useful to do short range scouting. (If you move the tonk too far from its controller, it will disappear) Tonk abilities:
1.Cannon – Basic shoot ability. You have to be facing the other tonk, this seems to have somewhat of an auto-tracking ability in that you dont have to have the enemy tank exactly in the middle of your screen to hit it.
2.Mine – Lays a mine at the location of the tonk. These are triggered even by your own tonk. They activates a short time after placement, so don't stay standing on it.
3.Boost – Increases movement speed
4.Mortar – This shoots a long range bomb at a fixed distance. It does splash damage, but its pretty damn hard to actually get it to land on another tonk. Its quite damaging.
Crashin' Thrashin Robot
These don't really qualify as minigame bots, but they aren't non combat pets either. When you summon one of these, I will start moving around the area you placed it in. It does not follow you. If it spots another Crashin' Thrashin Robot, the two will engage in combat and use a machine gun, lightning attack and a net ability to add some flavor to the fights. These bots are usable by anyone, the schematic is a random world drop in Burning Crusade zones.