Sentence structure and form

Is this sentence grammatically correct?

"The great sea creature came alongside Lotty and nudged him, so he climbed on board its strong back.

I don’t get what sentence structure the second part is.

"he climbed back on board" sounds correct to me, but why is "its strong back" added right after?

Can it have a place there? and, if yes, I’ll be grateful if you could state some examples such as the above-mentioned.