Also, spiders don't attack anyone.
because we are larger than them and they have smaller, weaker and easier to poison prey. Fixed that for ya. =P AFAIK, you are correct that IRL there are no spiders with a taste for human flesh (although some are quite aggressive and will bite first, then run).
However, since we are dealing with
fictional mutant giant spiders, approximations to actual large spiders wouldn't be too far out of line. Three main factors when determining what will attack what, in this
fictional zombie apocalypse (to maintain
suspension of disbelief):
What is its natural diet/prey(omnivore, carnivore, herbivore, etc)? Spiders are carnivores. They paralyze, then liquify and drain the fluids/organs of other living creatures, like a flesh smoothie...
What is the relative size of its prey, if any? Large spiders will eat other spiders and "small animals such as millipedes, wood lice (slaters), and even small lizards, frogs, and birds". They will not eat carrion. Yes, spiders mostly (some are MUCH more aggressive) run from anything significantly larger disturbing them. So anything alive, roughly 2-3 times their own mass, should be a fair game as prey, approximately. Good thing these spiders are only the size of large dogs and horses. O wait...
What level of starvation or disease are they suffering from, that might possibly get them to go after prey normally outside their palate? Well, in this case there is an unknown mutating Z virus (possibly the source of their size, but who knows) that causes a taste for human brains. Any spiders that size are going to quickly wipe out other appropriate sized prey nearby, so will probably be starving. If you've ever seen baby spiders hatch, they are numerous(hundreds), they are cannibalistic, and they are ravenous...
Sources: Google "large spider diet" or search Youtube for videos with "spider eating larger prey" or "aggressive spider". My faves are the trap door spiders. Enjoy and remember, IRL zombies don't actually attack anyone either (unless your name is Max Beauvoir and you reside in Haiti)!