The best replacement for lassie is flipper.

The best alternatives to lassie are rated as following:
★★★★★  flipper
★★★★  littlest hobo
★★★★  old yeller
★★★★  lad
★★★  rin tin tin
★★  cassie
★  lass
★  laddie
★  lassy
★  laird

Your alternative for lassie is not listed? Suggest!
Speaking about lassie, one probably shouldn't forget about timmy.

Other search terms related to lassie and their ratings are:
★★★★★ timmy
You have in mind a companion for lassie and it is not yet here? Propose!



Google Trends

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lassie
flipper
littlest hobo