PonyXL can do nipple penetration a bit, but I find this LoRA helps get some more consistent results. I made this for my "lipple" LoRA, so I skimped a bit on training data, but it can do cocks, tentacles, and sometimes fingers. It also seems to be able to blast cum/lacation out sometimes.

To use, a weight of 1 seems to work well for initial generation. Add the trigger word "nipple penetration" at a weight of 1-1.3, and "single penetration" or "double penetration" (ymmv). It's been trained on the prompt "a woman being penetrated by ________", so I'd suggest this line when using. For upscaling, a lower weight or removal of the LoRA can work alright since pony lightly understands the concept.