Soulmancer did some testing with the skill SummonResists, and was unable to find
any effect from it, so he replaced it with SpiritBlades. To give NecroPets resistances he modified their base creature type (monstats.txt) and properties (monprop.txt). He also tested elemental absorbs given to pets via those sources, and found that "absorbs" were only effective as a resistance that couldn't be lowered (so there was no cap at 40%, no life was gained for absorbs higher than 50%, and Conviction and LRes didn't reduce these).
Code:
NormRes* NormAbs** NMRes NMAbs HellRes HellAbs
Golems*** -25%/0% 33%/0% -33%/0% 50%/10% -50%/0% 75%/25%
Skelly 25%Phys none none 20%Ele 25%Phys 40%Ele
SkellyMage 25%Mgc none 25%Phys 20%Ele 25%Mgc 40%Ele
SkellyKnight 33%Mgc&Phys none 33%Mgc&Phys 25%Ele 33%Mgc&Phys 50%Ele
Blades**** 50%Mgc none 50%Mgc,-33%Ele 25%Ele 50%Mgc,-66%Ele 50%Ele
* Elemental resist (Ele) is resist fire, cold, lit, and pois. Resist physical (Phys) and magic (Mgc) damage are stated if applicable.
** AbsEle is just fire, cold, lightning, and pois. Absorb poison is just pois-length-reduced. These values are just resistances which can not be lowered, they do not act like Player-Character Absorbs. Summons can still have their resistances lowered, but if they "absorb" 75% of their element, then having -200%Res to their element will mean that they are only taking 75 from 100 damage (while a critter with 0%Abs would take 300 from that hit).
*** Golems are elemental-aspected: Clay=Cold, Blood=Lightning, Iron=Poison, Force=Fire. In the table above, the first number will be their aspect-number, while the second is their other numbers. For example, a Hell Clay Golem has -50%ColdRes, 0%Res(fire,lit,pois), +75%ColdAbsorb, +25%Abs(fire,lit,pois)
**** Blades, and probably all other non-necro summons, get 2%ResAll per skill level to a max of 66%. These Summons have the exact same resistance and absorbs properties as Blades (the "Summon" property in monprop.txt). For example, in Hell, a level 50 Blade has 0%ResEle, 50%ResMgc, and 50%AbsorbEle.