Sheaths don't have a set rating. Partially or fully visible sheaths on non-anthropomorphic animals are fine for "safe" as long as it's not brought to viewer's attention by means such as framing, posing, view angle, amount of detail, or level of realism (the more realistic the character overall is, the better chances are that a wee bit of a sheath detail will not elevate the rating; %I think a tracing of a horse photo as a MLP character should not have a different rating than the original would%). This is so that stuff like recoloured perfectly mundane irl horse photos and unedited show screencaps don't end up being non-safe.
I don't think it's ever been established by staff whereabout the lines between safe, suggestive and questionable sheaths lie, but I'd dare to say that at least featureless little bumps between the legs such as >>2607480 would be "safely" safe. (Incidental, and matching the cartoony style for the rest of the body: simple contour, no inner detail.)
I surmise that we'll probably end up with the same rules as with anus (explicit unless simplistic enough).