To me it seemed that in English one has to use 'its', not 'his', for all animals. But I looked it up right now and it turns out that when there is a personal relationship, as in 'Zofie's spider', one can indeed use he or she: https://www.writersdigest.com/editor-blogs/questions-and-quandaries/grammar/how-to-handle-animal-pronouns-he-she-or-it.
Yes, "lost HIS leg" can be interpreted as losing the only leg he had (left), but I wouldn't say it can mean ONLY that. Given that there are so many of them, it can certainly mean he lost any one.
That said, I tend to favor "lost A leg" for the main translation.
Spoiler Alert: That may open up the potentially two-fer worm can of "It should be HIS leg, because who else's leg could HE possibly lose?!?!?" and/or "We keep hearing that if no possessive is used, the whatever-it-is belongs to the subject, so why is it A leg and not HIS leg?!?!?"
(And this is just one more reason why some people don't like spiders.)