Theoretically, a very small animal, or one that is quite sick and unhealthy, or basically a new born…

They may be small enough and have weak enough skin that a longlegs could actually penetrate the dermis and actually deliver a potentially troubling amount of venom.

IIRC, small rodents are things that a longlegs could potentially deliver an effective bite to… but I am not completely sure where or how exactly to draw the line.

Basically, they have a decent venom, but their fangs are quite bad at penetrating most kinds of skin… they are geared mainly toward hunting other small insects… but its not 100% impossible to harm something a bit bigger.