Pets

Adoptable pets
Adoptable pets released so far include cats, large felines, dogs, and birds. Cat in a Bag, Big Cat in a Bag, Dog in a Crate, and  Bird in a Cage may be purchased in the Marketplace. Using the item will give you one of several different types, each at random.

They are familiars, not combat pets, and also do not offer any buff.

Selling
From the July 6, 2016 patch message:Lynea the pet collector in the Commonlands will purchase your excess adoptable pets for a unique currency. You can buy a random discontinued pet from her using this currency.

For selling her 10 adoptable pets, you get enough Adoption Coins to purchase one Random Pet in a Box.

Enabling old pet models
Enabling of the old pet models can be achieved either by entering configuration, or by making sure that this line exists in your eqclient.ini file:

Illusions
Since the November 16, 2016 patch, if you own the Familiar Key Ring feature, any illusion spells on summoned pets will persist when you zone.

Tanking behavior
Pet tanking behavior 1: Pets can tank. Over 90% of NPCs use this behavior since the December 9th 2015 update.

Pet tanking behavior 2: Pets can tank only if no player characters are within melee range. This is the way everything used to work prior to the December 9th 2015 update.

Pet tanking behavior 3: Pets cannot tank. A portion of the hate that pets generate is attributed to the pet's owner. NPCs that have the MotM buff use this behavior.

On progression servers:

The Cazic-Thule 2.0 bosses were using behavior 3. After the August 2017 update they will continue to use behavior 3.

The Gates of Discord bosses were using behavior 1. After the August 2017 update they will use behavior 2.

On non-progression servers:

The Cazic-Thule 2.0 bosses were using behavior 1. After the August 2017 update they will use behavior 2.

The Gates of Discord bosses were using behavior 1. After the August 2017 update they will use behavior 2.

For reference, here are the relevant bits from the Dec 9, 2015 update notes:

*** NPC ***

- Adjusted how NPCs choose a melee target in relation to player pets:

- - It is now possible for a pet to be the target of an NPC's melee attacks if it has the highest aggro with other players in range.

- - This behavior can be adjusted on a per NPC basis and will currently be disabled for all current and previous raid difficulty NPCs.

- - It is additionally possible for some NPCs to ignore the hatred generated by pets and instead attribute a portion of that hatred toward the owner. This behavior will be in play on raid targets on progression servers.