A bionicle seahorse is when a girl lays on her back and uses her knees to make fart noises, while her sexual partner engages in sexual acts on the back side of her knee, i.e. the fart center on the leg. Then the girl stands on her head and delivers a foot job, usually with a lot of force.
"Man that bionicle seahorse left my meat sack all red and tender!
She was knee farting everywhere!"