Consider Grizzled Angler. If I use the activated ability to transform it (assume the condition is met) in my first main phase, will Grisly Anglerfish be a new game object? It will be untapped on the battlefield correct? I can attack with it during my combat step, correct?
A permanent that has transformed hasn't entered the battlefield and doesn't become a new game object (C.R. 711.14). Thus, in general, you can attack with a transformed creature as long as you controlled that permanent (whether in transformed or untransformed form) continuously since the beginning of your most recent turn (C.R. 302.6). But remember, transforming a permanent doesn't untap it (C.R. 701.28a), so in this scenario, Grisly Anglerfish will remain tapped, rendering it generally unable to attack during your combat phase (not: combat step) (C.R. 508.1a).
EDIT (Aug. 1, 2019; Sep. 1, 2019): Clarification.
EDIT (Mar. 16, 2020): One rule was renumbered with Core Set 2020.
EDIT (Nov. 26, 2020): Some rules were renumbered in the meantime.
EDIT (Aug. 1, 2019; Sep. 1, 2019): Clarification.
EDIT (Mar. 16, 2020): One rule was renumbered with Core Set 2020.
EDIT (Nov. 26, 2020): Some rules were renumbered in the meantime.