Two characters inserting their tails into each other's pussy. Several other tags apply if this one does (feel free to copy and paste:)

mutual_penetration mutual_tail_fetish mutual_tail_insertion mutual_tail_play mutual_tail_sex mutual_vaginal mutual_vaginal_penetration penetrating_while_penetrated tail_in_pussy tail_insertion tail_sex

Here are the tags with links to their wiki pages (links are harder to copy and paste:)

mutual_penetration mutual_tail_fetish mutual_tail_insertion mutual_tail_play mutual_tail_sex mutual_vaginal mutual_vaginal_penetration penetrating_while_penetrated tail_in_pussy tail_insertion tail_sex