Torchlight Purple Cloud – Enemy Indicator Explained

If I remember well experience gained depends on monster levels and does not scale with your level. You are right, the experience that you get from killing mini-bosses is very high compared to that acquired from killing normal monsters.

There is also a type of fish that increase your XP gain by 20% for 300 seconds. Remember to eat one of that before big fights.

There are 3 sizes of pet-consumable fish:

• Regular fish transform your pet for 2 minutes and are fairly common.
• Big fish transform your pet for 1 hour and appear about 1/10 as often as regular fish.
• Giant fish transform your pet permanently and appear about 1/200 as often as regular fish.

The rarity of pet-consumable fishes are not affected by the tileset (crypt, mines, ruins, palace, etc) you're fishing in. (Character-consumable fishes are, however)

You can find a list of pet-consumable fish here on StrategyWiki. I don't see one that transforms your pet into an ogre...was it actually a Varkolyn?