The facehugger needs to be attached to the host in the proper fasion (ie: legs around head, tube in mouth, tail around neck, ect.) T-rex's head was the size of a human adult. A facehugger is the size of a human adult's head. There is no way an alien could infest a T-rex. If the animal is too huge, it just won't work. And a bat?! Bats are much too small to be impregnated by a xeno. In the uncut version of ALien 3, the Facehugger gets an Ox rather than a dog, which makes a million times more sense. Anyway, as Dave said, just leave the poor aliens alone. The designs from 3-onward were enough of a ization.
EDIT: Clearly, the swear-filter is still ON. LOL