Terrytoons entered into a new, but very brief, era in 1958 when animator Gene Deitch was brought in to run the show. He only lasted a year or two, but you have Deitch to thank for Silly Sidney.
Sidney was a neurotic elephant who was, as the stereotype would otherwise suggest, afraid of mice, but he also could get a little cranky at times. Lionel Wilson initially gave Sidney a voice based on Ed Wynn, then amended it twice before turning things over to Dayton Allen. Sidney, turning 55 this year, only appeared in a small handful of cartoons between 1958-61, and those toons were included on The Hector Heathcote Show during its 2 year run on NBC (1963-5).
My earliest memory of Sidney wasn't a cartoon. Instead, I seem to recall a book & record that came out way back in the day, and if memory serves, comedy legend Soupy Sales read the story. My memory might be a wee bit inaccurate, but Wikipedia certainly was no help.
Goinginsainsburys uploaded a pair of shorts---"Hide & Go Sidney" & "Tusk":
If the Wynnesque voice sounds familiar, other actors, such as Len Weinrib & Howard Morris, have done similar impersonations. Morris did so for the Mummy on Groovie Goolies, and I'm not sure which one did Mayor McCheese in those McDonald's ads in the 70's.