no

apelike or anthropoid primitive primeval primate

Probably. Yeti and Sasquatch are both described as hairy and apelike, so it's a safe bet that they are mammals.

Simon discovers that the apelike beast the boys fear is actually the rotting corpse of a dead paratrooper, whose body has become entangled in the rocks and vines on the mountain. This realization causes Simon to understand the true nature of the beast as a manifestation of the boys' own inner darkness and fears.

For the animal, simply ape ("ape gestures") or apelike. For the meaning to imitate, the adjective is aping or apish (which carries a derogatory context).