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On multiple threads here I've seen people make comments like, "X pet seems squishy compared to Y pet."
Do pets have an unlisted defense modifier that doesn't show on the lore screen or is this simply a case of people using a pet with low resist on a mob that uses that damage type, or using a pet that does X damage type against a mob that has high resist in that same damage type?
I think "squishy" is a registered trademark of Pawain Incorporated. You'll have to pay a small royalty when you use it, but it's set up to automatically deducted from your bank when you log in to UO. It's like the fee for stabling pets.
I don't think there is a "hidden" suck factor coded into some pets, but I think this is just the departure zone between "on paper" good, and "in the field" good.
I've had pets, that I thought would be good, and they had all the qualities of "on paper" good, but they sucked when I used them in the game and I booted them out of the stable. The opposite too, where a pet should be kind of a loser but turned out really good with its set of compromises. There's a lot of factors (Char build, game play, masteries, scrolls on the pet, and maybe even connection speed(?), and definitely more) that would contribute to this and I think it starts to exceed the ability to model and predict which is what "on paper" gives you. I guess if I had to pick a word salad word to describe it, I'd say "synergy".
I think it's important to listen to the "on paper" modeling and share numerical findings, but then build a prototype and see if that build works for you "in the field". This is also why it's helpful to hear other people's field reports and get some ideas on what to try that's best for all of your in game factors. A squishy pet is no fun.