尝试：

DT2 <- DT[ , as.list(quantile(x,probs=probs)),by=y]
setnames(DT2, c("y", paste0("q", seq(10, 100, by=10))))

#    y       q10        q20        q30        q40          q50       q60       q70       q80
# 1: b -1.281704 -0.8402934 -0.5251957 -0.2595748 -0.001625739 0.2526686 0.5251940 0.8379979
# 2: c -1.269750 -0.8323597 -0.5133207 -0.2478633  0.003413041 0.2598378 0.5353759 0.8477539
# 3: a -1.281899 -0.8389189 -0.5224092 -0.2573562  0.001186281 0.2542550 0.5244238 0.8401411
#         q90     q100
# 1: 1.284773 3.856234
# 2: 1.283465 4.322815
# 3: 1.273615 3.921410