Updated, most importantly with four additional pet forms. Also tweaked the stats on the original ones a little. Mostly the Dragonkin pet has been beefed up a little, to make him more viable for a number of levels.
Note that both the standard and Fishmash versions of Crouching Pet, Hidden Beast have been updated.
New pets include the Preying Spider, Widowmaker, Brood Mother, and Depth Dweller. First three are from my Champions Mod where they appeared as enemies, while the Depth Dweller is a modified version of the unused Torchlight boss Leviathan. Some of you may have met up with the Depth Dweller's big brother in the Starch the Monster Hunter mod

At any rate, as an unused monster it seemed like an interesting option for a pet.
The Preying Spider and Widowmaker are in general just stronger spider pets. If anyone happens to like the original spider pet, these might be good alternatives as the original spider pet is rather fragile and isn't of much use beyond the first few levels. The Preying Spider is faster, has a charge attack, and an increased critical rate. The Widowmaker is slightly slower, but stronger and does more damage. The Widowmaker's basic attack is 100% poison damage.
To distinguish it from the Widowmaker enemies in the Champions Mod, the Widowmaker pet is slightly larger than the enemy Widowmakers. It also has a blueish aura applied to it, both to help distinguish it from enemy Widowmaker, and to make it easier for the player to locate her or his pet in dark areas.
The Dark Priestess and Deathcaster pets provided an interesting alternative to the standard Torchlight pets, I found. Whereas the default pets are mostly "rush in and attack" pets, the Dark Priestess and Deathcaster are spellcaster pets.
The Brood Mother, then, is a summoner pet. Much like the enemy Brood Mothers in the Champions Mod, the pet Brood Mother can spawn spiderlings to attack enemies. The spiderlings all have a blueish aura, much like the Widowmaker pet, to make it easier for the player to recognise that they are not the enemy.
The Depth Dweller is a decently strong pet who can smash the ground and cause shockwaves that will damage enemy forces. There are rumours that it may also posses some other strange ability ...
While the Preying Spider and Widowmaker aren't particularly strong - though, as said, certainly a bit tougher than the standard spider pet -, the Depth Dweller and Brood Mother are certainly more useful than the standard Torchlight pets. That's because they're meant to be a bit more viable pet choices on higher levels. By which I don't mean level 90 on Very Hard, mind you. But they should prove a bit more useful than most pets, especially the Brood Mother with her summoning capabilities.
While on the subject of the Brood Mother, no, she's not a normal Tunnel Spider, if you want a Tunnel Spider pet, just fetch the
Tunnel Spider pet by Irrelevant, which is awesome and the reason why I don't have to bother with making a Tunnel Spider pet

A video with me showing the new pets, and some short bits of the Brood Mother and Depth Dweller pets in action can be found here:
http://www.youtube.com/watch?v=uexJo68r6r8Mind you that there are minor issues with the Widowmaker and Depth Dweller unit themes. Sometimes they don't appear, or stick to other pets. These aren't issues with my mod, they are issues with the game (or its engine). I've tried a number of workarounds to get around these issues, but haven't managed to find anything yet.
Usually, though, simply feeding the pet another fish or exiting the game will be enough to get rid of unwanted effects.
(actually, you can see one of these issues in the video. During the title screen shot of the Preying Spider, you can see a haze over the floor in the background. That's because the Preying Spider had for some reason gotten stuck with the Depth Dweller unit theme, and that haze is a vastly enlarged version of the blueish haze that should be under the Depth Dweller)