TV Tropes calls it "Fantastic Arousal" and there doesn't seem to be a tag for that concept. You've got tags for licking and playing with ears/tails/horns but no tag for the concept of a nonhuman erogenous zone being stimulated. Sure there's petted_silly but that focuses on petting something like it's an animal (while not applying to groping the tentacles of an alien woman or touching the wings of an angel or the gemstone of a magical creature) and mixes together sexual and nonsexual results.
The sexual version of petted_silly could be called heavy_petting.
