I was thinking for a futanari since you are thinking of exploring different types of cocks like horse cocks how about trying a Dog cock for a futanari.
For example the actresses will be Rae lil black and Rikako katayama. Rae lil black will be Rikako katayama dog that transforms when eating a special alien orb in her food bowl which transforms her over night into human form of Rae lil black but with a doggy dildo cock. Rikako wakes up to her surprise to see her puppy is now in human form or semi human form. Rae lil black now can show her owner Rikako her appreciation by sexually seducing and pleasuring Rikako with her throbbing doggy cock. Rikako is hesitant at first uncertain about it but gives in telling Rae she has occasionally stared at Rae doggy cock over time and masturbated fantasizing what it would be like with her best friend. So Rikako and Rae start having sex first oral sex with Rikako giving a bj to Rae with Rae howling in pleasure her internal feral beast is released. Then Rae puts Rikako in doggy position starts sniffing and licking Rikakos pussy and ass cause she can smell Rikako is in heat and ready to be mounted. Then Rae mounts Rikako in doggy position relentlessly pleasing Rikako.
Now I don’t know if you can convince Rikako or pay her extra to do anal sex with the dildo but if you could I would add that after Rae mounts her pussy have Rae mount her ass for Anal sex. Then after have Rae lay back and have Rikako bounce her sexy ass up and down on Rae in cowgirl position cause now Rikako is super in heat and excited. Plus it will be hot seeing Rikako bounce her ass. Get some really good close up shots of her ass cause she has a nice Japanese ass.
Then end it with Rae cumming and howling like a good girl. Then have the scene end with Rae in Rikakos arms and she’s petting Rae on her head and stroking her dog dildo cock saying you’re such a good girl You pleased your mama well you want a treat now.
Just an idea since you’re trying different types of dildos like horse cocks and dog dildos