zsh has a zmv utility for batch-renaming files:
$ ls
10_test.txt 1_test.txt 2_test.txt 3_test.txt 4_test.txt 5_test.txt 6_test.txt 7_test.txt 8_test.txt 9_test.txt
$ autoload -U zmv
$ zmv '(*)_test.txt' 'test_$1.txt'
$ ls
test_1.txt test_10.txt test_2.txt test_3.txt test_4.txt test_5.txt test_6.txt test_7.txt test_8.txt test_9.txt