Ovipositor I designed awhile back.

the egg sac contains small eggs that will grow within the host over the span of a few minutes.
the knot on it's neck puffs up, making it hard to pull the creature out from your body. Meanwhile the head pokes out to tease the host.

Edit----

Knoty is free to be used in your oviposition stories, and art! Just please tell me before jumping to use them. I'd love to see what you come up with with them ;)