Instinctively it looks like it should be pet then prey, since pet lasts 8 seconds (4 pulses) and prey lasts 6 seconds... However many rotation videos/guides seems to do the opposite. Is there something I'm missing?
ALWAYS pet then prey. We cast prey to boost the pet damage, therefore, we went as much uptime of both in a rotation.
If you go Pet then prey, your pet will tick for 6/8 seconds with prey active (3 pulses). If you go prey then pet, they overlap for 5 seconds (2 pulses).
Also, in any circular rotation, you want to cast DoTs in descending order of duration, beginning with longest and ending with the shortest.