Ranger

Notes on Pet Uses
There has been great debate on which pet is best, when to use a certain pet and the pros and cons of each ones abilities. This is a comprehensive list to help determine which a player would prefer.

Wolf/Blight Wolf

 * Pros:
 * provides a decent area of effect debuff.
 * Deals most damage over time out of the summons.
 * Fast, will reach a target quicker than NPCs and provide a distraction as you finish them off.
 * Cons:
 * Bit of a light weight in armor and requires some looking after in big fights.

Bear/Great Bear (equivalent to Bereskarn)

 * Pros:
 * First and foremost, a tank. It will take a sizable amount of damage before going down. Very useful at lower levels when an NPC tank cannot control all enemies or survive encounters.
 * at Master Ranger, can increase damage done by decent amount for a short period of time.
 * Warm and fuzzy
 * Cons:
 * Seems slower, if only slightly than the other summons.
 * On consoles, due to lack of third person overhead view, can block line of sight at times, making it difficult to target an enemy.
 * Large, which causes trouble to the movement of the party in tight spaces.

Spider/Poisonous Spider

 * Pros:
 * can be used instantly for interupting attacks or just keeping an enemy busy.
 * deals mild damage over time. Has a 12 second cool down.
 * at Master Ranger level, which will put an enemy out of commission for a fight. Especially useful against mage types.
 * May prefer to flank enemies. Could be due to lack of space on an enemy from other sides. (Needs to be confirmed)
 * Cons:
 * Considerably weaker than the Bear and only slightly weaker than the Wolf in armor.
 * Creepy looking, in a deadly, cute sort of way.

Exploits

 * It is possible to summon multiple pets provided you have enough stamina. Note that none of these methods work on the Xbox360 and the PS3 versions. To do this start summoning and instantly click on the next summon. It is only possible to have 2 summons out at a time using this method. As the game engine does not properly support more than 1 summon per character, multiple summons do not retain full functionality. Firstly, you will be unable to manually use skills from them (when you select them, the skills are greyed out, and say "Only main character is active while in camp"). Secondly NONE of their portraits will be shown on the top left hand side of the screen. If you want to select them, you must do so manually by left clicking on them. However, the "Select Entire Party" button still selects them as per normal. This also makes it more difficult to monitor their health and stamina status.
 * It is possible to utilize all of the pet's abilities as though they were a party member via tactics - the only way to do this is by clicking the "1 tactics slot added" notification upon summoning. The wolf has Dread Howl, Overwhelm, Shred, and Howl, and will use them via the tactics, effectively giving you another Mabari. The Bear and Spider have many extra skills available in the tactics screen as well, including overwhelm (Note: On the as of Patch 1.03 only the Wolf/Blight Wolf pets have extra abilites available in the tatics menu, while Bears and Spiders only have access to the 2 abilities of the respective master ranger versions). The tactics cannot be saved or accessed at any other time other than when first summoned though! This makes it rather tedious (especially since the summons disappear with every area transition without a special mod) but extremely useful in more difficult situations. The biggest disadvantages are that, unless you have a mod opening max tactics slots, only one tactic will be available. Further, the tactics page for the summon can ONLY be accessed upon summoning it. You cannot access it any way other than clicking on the "1 tactics slot added" notification. Don't miss your opportunity!
 * Prior to patch 1.02a, it was also possible (using Revival) to have all 3 summoned at once. Summon all 3 in sequence, and the previous animal will die as the next is summoned. Use Revival on the spot they died, and they will return to life, giving you 3 pets.

Unlocking

 * You can purchase the Ranger Manual from Bodahn Feddic at the Party Camp. Like with all specialization manuals, once you buy it in any playthrough, the specialization will be unlocked for all current and future playthroughs and the manual will be permanently gone from the vendor's inventory.

Reference
Dragon Age: Origins - PRIMA Official Strategy Guide