According to the list on the Torchlight wikia:
Normal - Transform for 5 minutes
"Big" - Transform for 15 minutes
"Giant" - Transform permanently, until another fish is consumed. If this is a non-permanent fish/egg, then after that one expires your
pet is transformed back into its original form.
Jackal Bass - Transform into a Jackalbeast.
Largemouth Bass - Transform into a Mimic.
Warsnout - Transform into a Warbeast.
Tunnel Shark - Transform into a Molebeast.
Web Fish - Transform into a Spider.
Flying Fish - Transform into a Swampflyer.
Bonefish - Transform into a Torso.
Crab Egg - Transform into a Crab Warrior.
Vampiric Spider Egg - Transform into a Vampiric Spider.
Jackalbeast: Increase poison armour by ~100% and grants a small amount of poison damage.
Warbeast: Increases base average damage by ~85%.
Spider: Grants immobilizing web attack.
- +25% physical armour
- +66% elemental armour
- +50% HP
- +33% damage (base)
- +90% Interrupt Resistance
Molebeast: 20% chance to Stun target for 3 seconds
Swampflyer: +33% Attack Speed
- +20% damage
- +60% elemental
- +25% HP (base)
- Shreds armour
- +100% physical armour
- +100% elemental armour (base)
- Attacks steal life for you.
Other fish (direct effect on pet, or misc.)
Frenzy Fish - For 5 minutes gain:
- 33% Faster Movement
- +33% Attack Speed
Gold Fish - Not pet food. Just sell it.
Prismatic Morey - For 5 minutes gain the following:
- +25 Critical Hit Chance
- +100% to Fire Damage
- +100% to Poison Damage
- +100% to Ice Damage
- +100% to Electric Damage