Peanuts Characters: © 1950, United Feature Syndicate, Inc.

The date is the year the image of the character first appeared. Snoopy has numerous copyright dates because the image evolved.

It’s when that pose was trademarked

The copyright date for a character is based upon a characters "look," - not when it first appeared (otherwise Snoopy would always have a copyright of 1950, and he doesn't).

A short, helpful, article:
https://collectpeanuts.com/2011/07/11/peanuts-collecting-101-dating-an-item/

Peanuts (Snoopy) has multiple copyrights, because his appearance changed so much over time. Ergo, both 1958 and 1965 easily could belong to Snoopy.

Peanuts Characters: © 1950, United Feature Syndicate, Inc. Charlie Brown
Peanuts Characters: © 1951, United Feature Syndicate, Inc. Schroeder
Peanuts Characters: © 1952, United Feature Syndicate, Inc. Lucy and Linus
Peanuts Characters: © 1954, United Feature Syndicate, Inc. Pig-pen
Peanuts Characters: © 1958, 1965 United Feature Syndicate, Inc. Snoopy
(1965 is when the television shows started.)
Peanuts Characters: © 1959, 1960 United Feature Syndicate, Inc. Sally
Peanuts Characters: © 1961, United Feature Syndicate, Inc. Frieda

Peanuts Characters: © 1963, United Feature Syndicate, Inc. Five
Peanuts Characters: © 1966, United Feature Syndicate, Inc. The Flying Ace
Peanuts Characters: © 1966, United Feature Syndicate, Inc. Peppermint Patty
Peanuts Characters: © 1968, United Feature Syndicate, Inc. Franklin
Peanuts Characters: © 1965, 1970, United Feature Syndicate, Inc. Woodstock
Peanuts Characters: © 1970, United Feature Syndicate, Inc. Joe Cool
Peanuts Characters: © 1971, United Feature Syndicate, Inc. Marcie
Peanuts Characters: © 1973, United Feature Syndicate, Inc. Rerun
Peanuts Characters: © 1974, United Feature Syndicate, Inc. Beaglescout
Peanuts Characters: © 1975, United Feature Syndicate, Inc. Spike
Peanuts Characters: © 1976, United Feature Syndicate, Inc. Belle

Peanuts Characters: © 1977, United Feature Syndicate, Inc. Molly Volley

Peanuts Characters: © 1978 United Feature Syndicate, Inc. Eudora
Peanuts Characters: © 1982, United Feature Syndicate, Inc. Marbles
Peanuts Characters: © 1989, United Feature Syndicate, Inc. Olaf
Peanuts Characters: © 1994, United Feature Syndicate, Inc. Andy