There were two times where a tiger in thea zoo ran out and bit someone to death, which happened twice.

Using “the" makes it seem like there is only one tiger in China.

In Hangzhou, (one of the best cities in China), three leopards escaped, and t. The zoo managed to keep them under wraps situation under control for half a month in order to continue selling tickets, andbut one is still missing.

Grammar: Using "and" to connect the last part of the sentence with the rest of the sentence contradicts the former part of the sentence.
Style: You can use commas around "one of the best cities in China" since it functions the same as the parentheses and it'll look cleaner.

15Fifteen elephants in Yunnan, as had to migrate due to severe habitat reduction and fragmentation caused them. OR Due to severe habitat reduction and fragmentation, fifteen elephants in Yunnan had to migrate.

Grammar: You want to group the sentence as an independent clause (fifteen elephants in Yunnan had to migrate) and a dependent clause (due to severe habitat reduction and fragmentation)
Style: It's up to the writer whether to write out a number or just use the number, but normally I see people write out the number. It's totally up to you though as long as you're consistent!

The most frightening thing is that all the way north, the possibility of finding a suitable habitat is getting"gets lower and lower"/"decreases".

Grammar: I'm not sure on the exact grammar here, but it's like "a bottle gets filled" VS "the bottle is getting filled" one states a fact where the other one implies that a bottle is currently experiencing an action.