I should also add that even though the equipment type (Ravager/Myrmidon/Evoker etc.) matters, it is more important to focus on the actual stats given by each piece of equipment:

For example, consider the following relics (showing just damage and strength for simplicity):
a lvl. 20 (legendary) Myrmidon relic, gives +110 damage and +12 Strength
a lvl. 20 (rare) Ravager relic, gives +68 damage and +11 Strength
a lvl. 30 (rare) Ravager relic, gives +95 damage and +15 Strength

Of these three, the legendary Myrmidon relic would be the best for a water pet, as even though the other relics are Ravager types, the Myrmidon relic gives the best damage and strength (it gives just slightly less strength than the lvl. 30 Ravager relic, but it more than makes up for it in damage boost)

Thank you, that's kind of how I've been using it thus far. I pick the one that gives the best health + str (if physical)/int (if magical) boost. It's worked kind of well, for everything except Lord Talon. :S