In a way, there are such characters already:
1. Bobo the Pig – fits perfectly, he requires regular "petting" 😘 for him to continue following you (each time adds more days of companionship).
Only, having kids with him is impossible, which is probably a good thing;
2. In a way, these 2 companions... They woud follow you, in exchange for mating. 🧐
Only, it's inconvenient that you can't replenish the days while on the run – and gotta return to the Fishtopia tavern.
I think it's more of a bug/incomplete develoment – would make total sense being able to prolong companionship while travlling, and the older guy even mentions this in his dialogues. That you would "give birth to many kids", and when disbanding him that he "still wants more seggs". Also during map encounter, slave hunters would see you wih a fishkind, and would assume you're his woman. 🤔
Heh, these two guys are quite hilarious to have around too – they would try to mate with any female NPC encountered in the game. 🫠
(troublesome, but often funny as heck)