Han and his crew were aided by "grizzled mercenaries" on the planet of Twon Ketee where the three rathtars were captured, contained, and brought aboard Han's ship by a retractable harness.
Lego Star Wars: The Force Awakens, which is actually canon and voiced by the cast of the film (including Harrison Ford as Han), has a segment devoted entirely to the capture of the rathtars.
Here is the opening crawl for that segment:
Hired by KING PRANA to capture
three rathtars, Han Solo and his
crew travel to TWON KETEE,
where a colony of the
dangerous creatures is known
Awaiting to assist them there is
a group of grizzled mercenaries,
hired for their expertise. But
despite their skills, the hunters
will have to tread carefully if
they are not to become the
According to one of Han's crew in a the lead-in video:
VAROND JELIK: Don't worry boss, I got us the best rathtar hunters in the system.
Here is a walk-through of someone playing through the segment:
At 12:25, we get to another movie segment where Han, Chewie, and the crew find a rathtar that has already been captured and contained by some hunters.
At 15:15, we see a harness being attached to the rathtar's cage before it is retracted up into Han's cargo ship.